I don't when it started but I have this low frequency hum/vibration coming from the rear. The volume cycles like a roar...roar type of sound and it is fairly subtle. I don't recall it being there last year when I had the stock gearing. I now have the 3.91 gears in it. I can feel it in the seat and it starts at about 60mph and ends at 65 mph and does not change with the clutch pushed in. I thought it might be my right rear wheel bearing because it showed signs of wear where the left on did not. I replaced the right side half shaft u-joints and the wheel bearing but there is no change. I know one of the wheels in slightly bent because the tire guy told me. Last year I had the wheels balanced and all seemed fine. The car has 56k miles on it.

Anyone have a clue what it may be?

 

ericcer

There is a thread on this and I dont remember who it was but several LT4's (mine included) have this low frequency hum or vibe around 60-70 mph that is not affected by engine speed etc.

This guy changed everything from shocks to bushings to ujoints to wheel bearings hunting this down , documenting , step by step on this thread. What he finally came to was the drive shaft being slightly out of balance. He had it balanced and fixed the problem. I am going to have this done when I change my catback and shifter , when it is easy to reach.

Hope that helps and try finding the thread ..... sorry I dont know more info about the title of it . Good luck..... :seeya

 

I just got back from the driveshaft shop. It turns out that the yoke on one end was not welded on correctly and the tube was bent. So I got a new one built and will be putting it back this weekend. The guy thought is was a factory defect and I agree. I am only put down 297 rwhp and I don't think that is enough to twist/bend the shaft.

 

Did that fix the vibe?

 

I had a bad rear wheel bearing, it would start a low pitched whirring kind of sound always at the same speeds, seemed like it was right around 65 and then again at 80mph. And would change if you were going around a turn at those speeds.

 

I put everything together and it is still there. The only thing left is to replace the drvers side wheel bearing and u-joints. I did the passenger side a month back.

 

If it has been there since the gears were installed, it is likely related to that. A damaged pinion bearing or improper preload on the pinion bearing could be to blame.

Have you tried turning the driveshaft 180 degrees and reattaching to the rear? The factory manual says that makes a difference because of the location of the counterweight. With a custom driveshaft, that will probably not be the case though. Good luck. :)

 

I solved the problem by unbolting it and rotating the driveshaft 180 degrees.


[Modified by ericcer, 8:57 PM 7/22/2002]

 

Excellent , ericcer.......I am going to do the same next month when I add a catback and shifter.....

Good info :seeya