1994 silverado lwb LS1 4L60E trans 3,500 - PA

[onefastaSSchevy]

ummm what was that sound at about 1400 rpms?

i think its the muffler, doesnt misfire at all, has flowmasters but they are old as ****. it didnt do that when i installed the engine and ran it open headers for a moment. i also had just started it for the first time in 3 months lol