I have a 1998 GMC Sonoma SLS with a 4.3 l V6 Vortec.
It used to purr, now it growls, sounds like a small diesel engine. Vibration can be felt (mostly in the dead pedal) and also vibrates/growls in park and neutral.
Just came back from usual oil change, but has never sounded like that before, usually a nice quiet motor. Also significant vibration at 90 kph, smoothing somewhat at 100 and up..Any ideas?
Per OP in comments (submitted to remove this question from the 'unanswered list' - please upvote Thor's comment, not this answer!):
As it turns out it's sometimes good to recheck your mechanics assessment so I got under the truck and the catalytic pipe joiner had disengaged almost completely from the joiner plate for the pipe leading to exhaust. It would stay more or less closed together at speed but when I slowed it would separate more lol. I've drilled out the bolts, put in new gasket and bolts, and she purrs like a kitten again. That said, will need new catalytic and exhaust in near future.
