I have a 89 GTA with a factory 5.7 liter and stock TPI setup. I bought the car 6 months ago with 157,000 miles on it but the motor was supposed to have been rebuilt around 40,000 miles ago. The car has run great and doesn't smoke. I have done some recent maintenance like new intake manifold gasket, injectors, plugs, wires cap and rotor. The transmission was starting to slip though and I just had it professionally rebuilt with new flex plate and torque converter. Since the transmission rebuild, I have driven to work (80 miles round trip) about 3 times when the engine started making a sound the seems similar to an exhaust leak at the passenger exhaust manifold, as if almost an open header. It only does it once the car has warmed up and been driven some. Other than the noise, I haven't noticed any other drivability issues, but it could have a power loss since I baby the throttle. I started the car yesterday morning and it sounded normal so I drove to work and have my service department at the dealership look at it and by the time I got there it was pretty loud. Since a tech wasn't able to look at it till we were closing, I just fired up the car and let it warm up until the noise started. He said it sounds like a problem with the bottom end of the motor or could be the bolts to the torque converter or flex plate loosened up and could make that sound. The tech won't be able to look at the car till Tuesday and will start by looking at the oil first, but I thought I'd see what you guys think? Since I just had the transmission out and blew 2,500 on a rebuild I suppose loosened bolts would be the best news as it is warranties. Any ideas?