Bad vibration (I searched)...please help!

[stang1986GT]

I have an 86, bone stock, 5 speed GT. On a trip this weekend on the Interstate around 75 miles an hour, I started getting a vibration and sounded like something was hitting the floor board. Only happens when I'm going up hill and feels like the drive train is coming apart. When I get out of the gas, it goes away. When pulling out or driving on two lane roads, it only does it going up hill around 1500 rpm or so. I tend to think that it's the U joint, but it sounds like the banging noise is coming further towards the front of the car, and I would think that if it was the U joint it would do it all the time while I'm in the gas. The tranny mount and engine mounts look ok. Anything else I need to check other than the U joints? No vibration at idle or anything (it's in the drive train), nothing on deceleration, and only when I'm in the gas, going up hill.

Haven't checked it out yet, but I need to know what else to check before I crawl under it. Drove it back home three hours and it didn't get any worse, but no better either.

Thanks folks, I appreciate the help.

 

[12Pack]

Check the driveshaft, my buddy had the same thing and we checked under the car and the driveshaft bolts were so loose we took them out by hand.

 

[Flavadave4]

x2 for checking the driveshaft

 

[stang1986GT]

Check the driveshaft, my buddy had the same thing and we checked under the car and the driveshaft bolts were so loose we took them out by hand.

Ok, are you talking about the 12 mm bolts on the back or the u joint bolts as well?

 

[SVT32VDOHC]

Ok, are you talking about the 12 mm bolts on the back or the u joint bolts as well?

Yes, check those, I had them come loose twice on an old 91 GT AOD....I had to use red locktite and a 3/8" impact gun, then they were good....they loosened up on my current notchback also..thats a 5spd...the red locktite did the trick again!

 

[stang1986GT]

Yes, check those, I had them come loose twice on an old 91 GT AOD....I had to use red locktite and a 3/8" impact gun, then they were good....they loosened up on my current notchback also..thats a 5spd...the red locktite did the trick again!

More to the mystery...now it's not doing it at all! (I didn't get to check anything last night...too late when I got in and I'm at work now). Doesn't matter what I do, now it's not doing it. Driving me nuts.