Saw like noise coming from rear, what is it?
 

For a while my car has had this rythmic sawing noise coming from the rear but I've ignored it because it was kind of intermittent. I thought something might be rubbing back there but I couldn't find anything other than possibly a slightly warped rotor and some play in the driveshaft (I can turn it about a 1/4 of an inch or so before it engages and the wheels spin). It's mostly between about 15 and 40 mph but it's shown up when I was going faster or slower, in neutral or in gear, and the loudness of it doesn't really have a pattern to it. Sometimes loud, sometimes quiet, but the sawing noise does get faster or slower depending on speed.

Any ideas?

Check the u-joints in the driveshaft, rear brake shoes/pads, wheel bearings, bent dust shield.

Dust shields and brakes looked good when I looked back there and I gave the rear wheels a pretty solid shake and there wasn't any movement, plus I had those replaced a few months back so I'd hope they're still good lol.

Looks like it's probably the U-Joint and according to the Haynes Manual I've got says something called a needle roller bearing in the transmission makes a grinding sound when worn and causes play in the shaft so that sounds about right. But the noise is coming from the rear, not my transmission (or at least I hope it isn't)

Guess I should check the transmission fluid to see how that's holding up and be able to cross it off the list of possibilities. Apparently any Rx7 past '82 with a stock driveshaft has non-servicable U-Joints and needs the driveshaft replaced along with them. Maybe I'm wrong, but I've got a hunch trasmission fluid is cheaper than a $400 drive shaft. Any excessive amount of metal flakes in the fluid will probably point to what my problem is.

All first gen rx-7 u-joints are replaceable. The ones they label unreplaceable are called Staked U-Joint. Do a search in the first gen section and you will find several posts by myself that lists what is needed to replace them.

I say its the driveshaft also, once I had heavy vibration/ noise that can be felt from the shifter and rear of the car. I replaced my motor mounts and tranny mount thinking it was that. It ended up being the driveshaft.

Have you checked your rear end fluid level? Has it ever run dry? What about cupping tires; this one gets overlooked all to often.

With noise at any gear/neutral and no shaking on accel I'd rule out driveshaft forward.

Check the u-joints for axial play and binding. Is there a vibration at speed that accompanies the noise? All other above statements covered everything else really. You need to get that driveshaft down and check it out.

I have to also jump on the U-Joint bandwagon. It's the most probable issue from the symptoms you describe. They aren't that bad. If you wait too long, it can leave you stranded in less than ideal circumstances.

Just checked the fluid in the rear end and it's completely full. The oil seems to be in pretty good condition too. All or my tires are in good shape, maybe some slight extra wear on the outside up front but no cupping or flat spots.

None that I can tell, but it is certainly possible that they are there and I've just tuned them out at this point.

So I'd guess it probably is the U-Joints based off of what you guys are saying, and the part number from rockford auto is 430-10A, right?

From what I'm getting it's just the same as replacing any other u-joint with a C-clamp on the inside. Is that the case or is there some extra step I'm missing?

I just went through this. My car developed an annoyintg vibration at speed and for awhile I thought it was a tire/wheel issue. Recently I dropped the tranny to do teh clutch and on inspection the front ujoint had on leg that was frozen. When I popped the cap off of it just brown powder came out and no bearing left inside. The spindle for that cap looks all chewed up. Put everything back together and its smooth as can be.

Could be the driveshaft.

That's the right part number.

Where is mine I had done. https://cimg5.ibsrv.net/gimg/www.rx7...efffe610b7.jpg

I am going to say rear end. When i first got my 1st gen, it had a similar noise. Me being young, kept driving it until it chewed up every bolt of the ring gear, except one and the diff locked up on me, luckily while pulling out of the driveway and not at speeds

Ordered the U-Joints yesterday, should be here in a few days. Hopefully it's as easy as YouTube makes it look

I am suspicious of the rear end, but considering how long I've been ignoring the sound and all the stupid things I've done in that time I think it would have blown by now. I'll be changing the fluid in there too just to be safe so I'll get a good look at it then.

Alright, so U-joints came in a while ago but I couldn't get them off an the shops around here refused to pop them out and put in the ones I ordered. The only offers I got were to have a new drive shaft made. While expensive, it's cheaper than Mazdatrix and easier than trying to find a junkyard shaft and hope it's good. There's just one thing, it looks... short? On the transmission end I've got about an inch of the bit that goes in the transmission sticking out. Is this normal?

Previously there was some sort a dust cover or shroud covering it up so I couldn't really tell just how far in it was. It was welded onto the old driveshaft and the guys who made the new one insisted it was not needed.
https://cimg0.ibsrv.net/gimg/www.rx7...1b2d4e4088.jpg

As for the shaft, I know it sticks out some. How much, not sure, but it does stick out allowing for movement with the suspension.

I may have missed it, but did this fix the problem? If not I'd go with the rear end once again. Intermittent grinding (or sawing noises) can occur from worn gears or faulty bearings allowing the gears to float. I recently had similar problems, although mine was more of a random clunk here and there. Ended up the pinion gear was destroyed and my axle bearings were toast.

Alright, that's good. Unfortunately I can not say this fixed the issue completely. There are fewer grinding noises, but in the few days of test driving there definitely is still some coming from the rear. There is also a noticable clunk if I let put on the clutch too quickly and when I lightly get back on the gas while coasting. I haven't cracked open the rear end and looked at the gears yet but I am starting to feel less than optimistic about their condition and that of the bearings.

It looks like all 4 bearings will be a bit over $200 but I haven't checked how much a new rear pinion seal would be or the cost of all the tools. I've found a good guide or two here to rebuild the differential and it seems, with the right tools, that It's easy enough. They say it costs way less to do it yourself than to pay someone to rebuild it but I can't find a total cost anywhere. Does anyone have a ballpark number for the cost including the purchase of tools? Thanks

Mazdatrix has a new set of ring and pinion gears listed for $700 so hopefully those aren't destroyed.

IF you decide to crack it open:
If the gear and bearings are in decent condition, I would leave the pinion alone. There is some sort of crush sleeve that must be replaced if you remove it (or so I've been told). Personally, I couldn't remove the bolt on the end of the input shaft and instead stole the new diff from my parts car. Check for play or sounds in the carrier and pinion bearings. I would suggest replacing the carrier bearings regardless and of course the wheel bearings while you're at it. To remove the bearings I used a cut off wheel and chisel, but you have to be careful not to damage the shafts. On the carrier I installed the new bearings myself using the old inner races and a block of wood with a hammer. If you go that route make sure they're going on straight otherwise "things" can happen. As for the axle bearings, my local shop put them on for free so your mileage will vary.
While it's all apart clean it up good and make sure there aren't any rouge metal chunks hiding away. I hit mine with a coat of paint for the exterior while I was at it.

I did it without any special tools and there' a good chance your gears are fine. If it's not your DD I would put it on blocks and take a look.
Good luck!