Rear Sway Bar Interference
 

The rear sway bar on my 81 has an offset and it rests against the transmission mount. The sway bar on my 82 has no offset and is straight across and has ample clearance. Occasionally at idle I can hear a knocking sound that I think is coming from this. I am not the familiar with the different sway bar designs, what do I have here?
http://forums.pelicanparts.com/uploa...1585432668.jpg

The one with the bend in the middle is probably a 21mm bar from an '87-'89 car with the G50 transmission. Sway bars from 915 equipped cars were straight. The only straight 21mm rear bars were from 1986.
For what it's worth, i have a similar bar from an '87 in my '84 911, and it just barely clears - I haven't had any problems with it knocking. But these cars can vary a bit, so YMMV.

It's meant for a different purpose, but Tarett makes a spacer that goes under the rear sway bar brackets to provide more room. If your bar is hitting the trans mount it could help solve the issue.

https://www.tarett.com/items/911-products/911-suspension/911-swaybars~drop-links/spacer-911-rear-swaybar-bearing-block-g50-or-930-trans-ea-rblkspr1-detail.htm

Maybe your swaybar mounts are slightly tweaked, or bushings are shot. I run the same bar without problems.
Also check your trans and engine mounts, maybe your drivetrain has sagged a bit.

Thanks guys good input. I neglected to note this is a 3.6 conversion with spacers under the engine mount. I had things apart recently and the transmission mounts and bushings are almost new. I may give the Tarett spacers a try without much to lose.