Car making noise during acceleration

[coolhuh]

My 1986 Alfa Spider makes a vibrating type of noise during acceleration. If I take off slow and don't give it much gas it does not make the noise. If I give it about half throttle of more it makes a loud vibrating noise in the front half of the car. It usually starts at 3500rpm and up. Any idea of what this could be. I know the motor mounts were changed about 5K miles ago.
Thanks

[Davy13]

The likely culprit is either the drive shaft support bearing, a u-joint, or the rubber dough-nut that connects the shaft to the transmission. Do a search of this site, as I know that this has been covered a few times, so I am sure you will get a lot of info here...

[ocduff]

Look under the engine at the oil pan guard. Is the engine less than an inch from it? If so, the motor mounts need to be replaced. Check you mounts visually - if there's cracks or oil or bulges or sags, they need to be replaced.

Otherwise - check the heat shielding between the catyalytic converter and the underbody - if it's touching anything, adjust it so it doesn't. Also, there is a bolt that attaches the cat converter to the transmission mounting plate. Check that is there while you're down there.

[Ren]

I had a similar problem. In my reply I assume that the vibration is only present when the car is moving? Also the vibration comes in at 3500rpm and then goes away at higher rpm?
First of all make sure that the engine mounts are secure and have been fitted correctly. Then check the drive shaft bearing support. If these are ok....and you are 100% certain they are, check the front end of the drive shaft. You will find in the front end of the drive shaft an allignment bearing. This was the cause of the vibration on mine. What appeared to have happened was, that as the engine mounts wore, the engine sagged. This increased the angle on the allignment bearing and altered the pattern of wear. Once the the mounts were replaced and the engine was in its original position the vibration became apparent due to the wear on the bearing. The bearing is a sleeve type and made of bronze ( phosphor bronze). I removed the shaft and had the bearing machined. Virtually all the vibration has gone. Note, it seems that it is advisable to lubricate this bearing on a regular basis. Replacement bearings are as rare as hens teeth! A good machine shop can make you a new one. If you have a copy of the Alfa Romeo Owners Bible by Pat Braden, there is a picture of the shaft and bearing on page 169.
Finally, before going down this route, do make sure that all the other points mentioned are checked first. What happened on my Spider is a very rare case! Hope you get to the bottom of the problem... Good Luck!!!

[gamebowser]

It could be the exhaust manifold bolts are loose. Check them and tighten them. Also, check the bushings that connect the exhaust system to the transmission in order to dampen vibration. If they are bad, you can get vibrating/rattling. This can crack the manifold.