Ok so 3800sc out of a 1997 riviera I finally finished the install and got it running I did a test drive with it the other night stock open manifold out the trunk to see how it would go and now all the sudden I have this what could this be. I got the motor from a bone yard which showed 150k on the odometer. Heard it run didn’t make any tapping but now it sounds like it’s top end knock

Sorry to hear about the knock. you could drain the oil and check for metal or silver colored oil to start with. Next pull the valve covers for inspection and see what you find?? I was going to look at a 1998 buick 3800 super charged with 157K . I lost out on it. but I was going to rebuild it. Hope this helps

Get yourself 2 1/2' of 1/2"or 3/4" hose and a large funnel. Using duct tape fasten the hose to the funnel. Now start the engine and use the hose as a stethoscope, the funnel being the ear piece end. Probe around and you will locate the noise.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "