aifilaw

1-2 shift is fine,
2-3 shift makes a loud squeak/squeal just when it shifts for a split second
3-4 shift is fine

this only happens at 70% throttle and above, below that, nothing. Brand new tranny.
all shifts happen at the same speed/time as before, and the noise is definitely coming from the transmission area.

pyro719

Frank should be able to answer this since he built it.

Billy Biker

It's belt slip. Now that I've firmed up the shifts alot with my programming mine does it too. Belts brand new and tensioned properly...I just ignore it. Here's some info from another site for you. Lots of good stuff here--
http://www.f-body.org/oldfaq/html/faq/toc.html
If you've ruled out that it's the rear tires chirping, then it's most likely belt slippage. This is common on the Corvette LT1s and even some of the later L98 motors. On hard shifts (auto and manual trannys), the shift in the speed of the pulleys will throw the belt and hence the screech of it slipping. It's pretty harmless, although it can be annoying.

You can check with your dealer as there is a Technical Service Bulletin (#476109) which calls for replacement of the belt tensioner with an improved one (part #10238703). Some people have also replaced the A/C pulley, (part #2724717) and the belt (part #12550149).

Others believe the power steering pump pulley to be the culprit. The do-it- yourself solution is to raise the vehicle and support it with jackstands (or ramps). Either look to make sure you have an underhood sticker that shows the routing of the belt or make yourself a picture. Then get under the car and use the appropriate socket and "tighten" the center bolt on the belt tensioner pulley. As you "tighten", the bolt shouldn't move, but the tensioner will swing out, allowing you to pull the belt off with your other hand. Then take an old toothbrush and soak the head in alcohol. Then, for each pulley, spin it with one hand and hold the toothbrush against the surface of the pulley where the belt normally touches. Repeat this until you are pulling back a relatively clean toothbrush on each pulley. Since you can't turn the main crank pulley, just run the toothbrush around it until the pulley is clean. Note that you can accomplish this cleaning with the belt installed and the engine running if you know what you are doing, but this can be EXTREMELY DANGEROUS if you don't have practice and a steady hand because IT IS EXTREMELY EASY TO GET THE TOOTHBRUSH CAUGHT BETWEEN THE BELT AND A PULLEY WHICH COULD CAUSE SERIOUS INJURY. Try the first way to be on the safe side. After the pulleys are clean and the factory belt is off, you can roughen up the PS pump pulley by simply running sandpaper across the PS pump pulley from above. Watch your hands though, because those hoses up there can be HOT. Just scrape the sandpaper across the top of the pulley in a motion from the front to the back of the car. The trick is, you want to roughen the pulley, putting scratches in it in the opposite direction of the belt's travel. Scratch it just enough to dull the surface all the way around. Use a paper towel damp with alcohol to clean off the pulley all the way around where you sanded it to get the grit off. Finally, install a new belt (preferably, Gates K060637, 21mm x 1635mm) and make sure the routing is correct and the belt is centered on each pulley. Lower your car, start it up, and make sure the belt is staying on. Then you can hit the road and open it up, enjoying your quiet shifts (hopefully).

12SCNDZ

These cars are notorious fror belt squeel at the shift points. I get alot of questions about this one.

Frank
CPT

aifilaw

I'll buy the belt squeal theory as viable and that' its coming from the front clip instead of underneath.
The big question is, why doesn't it occur in 1->2 shift, or 3->4 shift, but only in 2->3 shift?