About a week ago, my car all of sudden made this loud rattle while driving down the highway. I was able to drive it back home without issue, since then I have not been able to narrow the cause.

The rattle rises and falls with rpm while stationary. Clutch in/out and transmission in/out of gear doesn't change it. It seems louder inside the car, which is why I thought it may be transmission related.

I thought it might have been an exhaust leak popping, but I've just checked the header and silencer bolts, no change. Heat shields are secure. I've taken the belt off the alternator and water pump, both spin smoothly.

Power doesn't seem affected, it accelerates normally. Shifts fine too.

Any ideas?

Video from inside. I recorded it from inside the engine bay, but the fan and intake noise make it barely audible.

The engine is an S4 13B, with dual side draft Webers and RB Streetport exhaust system.

 

Sounds like a mani leak on one rotor. Pics of engine? I want dual webers...

 

You kinda answered your own question if it still does it with the clutch pushed the drivetrain is disengaged from the engine not saying its and engine rattle but its not the drivetrain

 

Did more poking around, I thought it may be something in the exhaust like a loose housing insert or something, so I took off the header and didn't find anything.

I'm still thinking it's some where at the engine+transmission union. I drove it around the block and discovered the rattle gets louder on the overrun. I'm not feeling much any vibration, it's just a rattling noise. So I'm thinking I'll have to drop the transmission and look around in there.

Wouldn't a leak that big make it idle weirdly? I checked the bolts and sprayed carb cleaner around the manifold anyway, no leak was found.

There she blows:

It all started when I found that manifold on ebay for a ridiculously low price. It's for a 4 port, so I made an adapter. Tuning was a little weird because the aux port sides draw more air than the middle ports. Interestingly, the fuel economy improved a couple mpg than with the single DCOE or IDA.

 

i want...

but i meant exhaust mani :P

 

Check the exhaust connection flanges for warping. That could be an exhaust leak source.

 

Get a stethoscope, harbor freight a cheap one that's effective enough. Just move it to different parts of the drivetrain and listen to where it gets louder.

Just a shot in the dark here, but I'll guess waterpump.

 

I pulled off the header. Flange is straight. I couldn't find any holes, I put a really bright LED light into the tubes in the dark and no light shown through.

Then I reinstalled it with an unused gasket from my Atkins rebuild kit. No change, so I'm ruling out an exhaust leak.

 

Set your idle really really low on your carbs and tell me if the sound gets worse while sitting in the passenger seat....

 

Does it go away with the clutch depressed?

 

The sound is the same between normal and lower idle.

It makes no difference.

I took out the clutch fan so it's more audible in the engine bay, here's me giving it 2 revs. (12s clip)

 

Check the trans mount... or possibly one of the motor mounts.

 

Not related to this but, you know that those vents on the OMP oil injectors are sucking in unmetered air into the engine. Those need to be joined together with a 4 way splitter and fed to a metered vacuum port after the carbs. It will improve signal at the boosters and probably improve your mileage even more.

 

Finally became warm enough (for a Texan) to crawl under the car and change all the mounts. I did the engine and both transmission mounts (middle and ends of the crossmember). No change.

Most of my searching on the forum indicates a rattling or knocking sound indicates a bad engine bearing. I drained the oil again and didn't find large quantities of copper. What traces I did find I really couldn't distinguish from "regular crud". That's not to say it's not a bad bearing.

Also did a compression check, with the button held down both rotors got even bounces to 60 psi. I'm thinking the engine's just on its way out.

The training manual for the FC says the hoses lead to atmosphere, and in the fuel system diagram ahead of the throttle butterflies.

 

Regarding the compression test: the car hasn't been driven much, so the cranking speed during the test may have been a bit low.

 

Mazda trix explains on website. Mine was vibrating on accel and coast. None on decel. So i just ordered a 300 dollar driveshaft. Am i a 100% certain? No . But i know my brothers rx7 death by rattle. So get it fixed soon. Already had a starter bolt come out twisting my starter and breaking it. But i also commute an hour to work and an hour home everyday in a car thats 30 years old.

 

sounds like spun bearing/bearing knock to me Does the noise seem to be coming from the front or rear of the engine, or central to the engine? also, what are you running for a flywheel? I ask because maybe (if you are running an aftermarket flywheel) the bolts for the pressure plate or counterweight may be backing out or loose, if the noise is coming from the front and all the belt driven accessories seem to be ok, then maybe something is awry in the front stack (like the oil pump drive chain)...you could also run the engine shortly with out belts on it just to eliminate accessories from the equation

 

I did run it without any belts a while ago and the rattle was still present.

The noise seems to be coming from the rear.

It has an aftermarket steel flywheel. Forgot to mention that I also took down the transmission to check out the clutch, since that's what I thought it was, but everything back there is tight. So it's definitely inside the engine.

 

this chart may help narrow down the cause
GL

Trans / Diff Troubleshooting

 

my bet would be a spun bearing as well. I just expeienced this with a friends car. But the rattle did change pitch under load

 

Thread closure time! Everyone who said spun bearing gets a cookie.

Took me over 6 months, but I got to putting together a 12A from parts I had lying around and dropping it in. Then I tore down the 13B that was in the car:

Rear rotor bearing spun out, broke several teeth on the gear.



Front rotor looks okay apart from its bearing, still gonna check the seal grooves. The irons look okay and aren't scratched up, but also need closer inspection.

The only internal part that came out unscathed was the front stationary gear.

I'm not sure what the cause was. Oil pressure was good (stock gauge), but if it wasn't, shouldn't the front bearing go out first because the rear gets oil first?
Can they wear out due to age and mileage? Definitely was not over revved, and the bearing spun cruising at 70 mph on the highway.

Both rotor housings show chattering, the rear a little more than the front:


For posterity, I was using Atkins regular 13B apex seals. It clocked almost 70,000 miles. Daily drove it for 3 years then used it for weekends and a few track days. The height of the shortest seal after all that is .290", tallest is .298". (Mazda's standard heigh is .315", lower limit .256")

My concern now is the 12A I just dropped is using the same oil cooler that I didn't flush out. ( I thought the 13B was dying due to apex seals making the fluttering noise like Kentetsu's engine way back.) However, I didn't see any fine metallic/copper particles in the oil pan, pick up tube, or the oil pump. The oil pump rotors were still smooth and not gouged up. There were big chunks of probably rotor gear in the pan. I'm hoping changing the oil every 100 or so miles 3 or 4 times will thoroughly clear out the cooler. The new 12A has 150 miles on it now.