I have an issue i have not been able to figure out yet and was hoping you guys could help me out. Basically my car is getting a pretty good i would call harmonic vibration at about 60 mph. About 3 months ago i had a guy install 3.73 gears in the car which howl a little more than i would think they should, also about this same time my built tranny busted the mount and i drove it about 3 days with a horrible knocking sound when the car would shift hard from 1st to 2nd gear. I did install a poly mount at that time. It seems about then is when the problem seems to of began. Whenever i get up to about 60 mph i hear or feel a vibration that seems to be coming from the right rear of the car. The ujoints were replaced about 5 months ago so i raised the car on a lift and turned the tires and thought that me and 2 buddies saw a slight distortion to the driveshaft when the tires were turned. I replaced the driveshaft with another steel one and the problem is still there. I also changed the back right wheel with another one and had it balanced with no luck. I have put the car on jack stands and got the car up to 85 mph and at about 60 it starts the harmonic issue... I am thinking it is something with the rear end but really am stumped on this one. Please let me know if you have any ideas. Also the vibration seems to be noticable at lower speeds when i gun the car off the line... Thanks for any help on this issue.I am supposed to go to the track this friday night but really dont want to with the car not right. It vibrates pretty good at high speeds.

 

I have this same problem. I think the guys who set up my 4.10's screwed up the install. They have been noisy and had lots of vibration from the day I had them done. At the time I had the stock steal driveshaft, so I figured most of the vibration was coming from that. After changing over to an LS1 aluminum driveshaft there was some improvement but they were still vibrating and noisy. So I would also like to know if there's any way to solve this issue wihtout having to buy a whole new gear set up.

 

It could be vibration from the poly mount via the transmission under brace.

SteveC

 

so do you think i will have to go back to a rubber mount or just look at install of the poly one?

 

I had the same problem with my steel driveshaft as soon as i changed my rear end from 2.73's to 4.10's. I balanced it twice and the problem was there, replaced the joint the problem was still there i had the poly mount as well. Well i figured out that when you change your rear end it spins more meaning the driveshaft due to the gearing i got a aluminum one had it balanced and the problem went away. The steel driveshafts are notorious for vibration issues and ive heard from several people after rear end changes were it gets worse. Anyway wether my guess was right or wrong about the amount of times it spins is mulitplied due to the rear end change hence it will mulitply the vibration that you had or hardly had into a bigger deal if its like a harmonic vibration as i read above it should be from something thats rotating should be your driveshaft my guess could be off thats what it was for me and i narrowed it down to the steel DS is POS again im just saying what worked for me and i luckly guessed what was the problem since no one else new. Either way it worked for me

 

I had a thin piece of soft rubber installed between the poly mount and the transmission cross brace, it helped alot.

SteveC

 

Im having the same problem and started after I changed the rubber tranny mount to the poly one. I thought maybe the guy didnt preload it. But SteveC might be on to something. GOOD CALL!

 

Thanks steve, i am going to try that out. I may try a piece of tire sidewall or something, not sure what else i would use. i may just go back to a rubber mount if i cant get it to work .