You're most likely getting a harmonic noise through the drive line because either your pinion to driveshaft angle is incorrect, or the driveshaft to trans angle is incorrect, or a bit of both.
Because it's only when you're on the gas it means that the pinion is lifting and changing the driveline angle, when you get off the gas the pinion drops and obviously the angle is better at this point as its quieter.
If you think your pinion angle is about right try raising or lowering your trans, or engine. It'll either get better or worse, then at least you'll know what direction you need to go.
Sometimes it can take a lot of trial and error to get it right. Every car is different.

Note: driveshaft unis need some angle. No angle at all creates the worst harmonics.

 

ok so i dropped the motor about .5" and noticed the output shaft on the trans is raised a little bit to give more of an angle on the driveshaft. i now have noticed the noise is after about 3000 rpm and it is more noticeable in fifth only on gas. how do you guys setup pinion angle? i feel i have tried almost everything to get rid of this noise.

 

There is no easy way to set it up, it's only through trial and error that you'll get it right. Some cars can be really frustrating, and others really simple. Make one change at a time. If it worsens the problem you'll know you need to make the change in the opposite direction. If it helps the problem then you'll know your going in the right direction. Even if you don't get it perfect, try and get it as best you can under power and don't worry about the decel too much.

If you're unsure about the condition of your d/shaft it might pay to get it checked. The crosses need to be in good cond and it needs to be fairly straight and in balance.

If the driveshaft is quite long that can also cause problems. Rx2's are a good example when using a 1 piece d/shaft. The longer the d/shaft, the larger the tube diam should be. Yours should be 3in O/D if it isn't already, especially if you have a low diff ratio. The lower the diff ratio,the higher the driveshaft revolutions, increasing the likely hood that 'whip' will occur.


One particular car I could never get right, even with a brand new driveshaft. I tried every possible drive line angle until it was as good as it was going to get. In the end it turned out to be the cheap trans yoke the d/shaft manufacturer had used. Instead of fitting the spicer end I asked for they used a chinese item. The spline was too big making it loose on the trans output shaft, which in turn was setting up a harmonic at high shaft speeds. (5.29 final drive). As soon as a Spicer end was fitted the harmonic went away completely.

 

Pinion to d/shaft angle starting point wants to be roughly 2-3 degrees, but each car wants something a little different.

If you have a 4 link with adjustable bars you can set it that way, if you have leaf springs you can use wedges between the spring and diff. If you have an independent rear end then you'll do it by raising/lowering the engine and or trans. (you may also be able to shim the diff nose mounts)

In your case I would start by giving the pinion to d/shaft a little more angle.

 

actually the driveshaft is brand new and i got it rechecked for the balance just to make sure but i will check the crosses again. they do move freely last i checked.

 

hey pdf if i try and match the angle coming out of the trans on the pinion side it would be nose up. is that ok to do? i heard this would make it harder for traction.

 

It's common to have the nose up in relation to the ground, it'll probably be on a similar angle to the trans. I don't know if it'll affect traction, if it does I don't think it would be significant. In your case you may not have a choice. Just try it and see. If it solves your problem then later on you can try dropping the nose a little and see how it affects hook.

 

ok back from the dead.

i tried everything i could to get this noise to go away and still the same result.

i noticed that after adjusting the car and with it still in the air i would put it in fourth and/or fifth gear there a lot of vibration like something is not straight.

has anyone gone through this before pdf, allen, manny,.... someone lol

its really starting to bother me.

 

There is alot of advise and pretty much everything is covered here in you thread regarding your problem. Please post a list of exactly everything that you have done and checked so that we can narrow the problem down for you.

 

ok well i have done everything allen has advised me to do as far as adjusting the links to get pinion angle down and centering the wheel. i had made adjustments to motor and trans mounts as pdf suggested to see if i can adjust the angle of the output shaft of the trans. still getting vibration and noise.

i know a lot has been covered and apologize if this sounds repetitive. believe me i appreciate everyones help with this. just yesterday driving home from church my uncle told me the car sits uneven. the driver side sits lower than passenger side and that the passenger side wheel sticks out more noticeably than the drivers side. im starting to think that the bracketry on the grannies rear end is not welded correctly.