Grinding sound during startup/low rpm

[db6_ix]

TLDR; possible misfires, shaking, but mainly grinding of metal sound coming underneath car, what is it?

I have a 1998 Q45t, just got it not even two weeks ago or so but I’ve realized there a lot of problems unseen (my poor judgement). Just recently, as of two days ago, the car began to sound louder with this metallic grinding sound coming from underneath. This only happens during low rpms, when I’m stopped at a light, and letting it warmup (below 1k rpm). I have no idea what it could be and can’t find a definitive answer.

It also would be good to mention that it is also coupled with shaking, I’ve been told it could be minor misfires, when I am also at a stop or low to higher rpm. I’ve attributed this to the fact that, despite the car stating “PREMIUM UNLEADED ONLY”, I followed external family advice to put regular for two full tank cycles. Never again, and I own up to it.

[fontana dan]

Two tanks of regular fuel shouldn't do any huge damage to your engine under normal conditions, but you probably want to chase it with some premium.
Can you make a video of the noise with the hood popped? It could be coming from something on the accessory belt. When you say "under" it makes me think bell housing/transmission. You've checked the engine oil and transmission oil level? Any leaks?

[98_Q45]

It could be an exhaust hangar of sorts, I know I had that issue on another vehicle before. Hard to tell without you going under and checking while engine is running (unless it’s only happening while driving).

I’ve run regular a few times and it doesn’t do anything bad, however the accumulative effects is likely a reality. Mid grade is a better compromise, but I’ve been in some areas where $1 extra for premium or .50 more for midgrade when gas is already $4-$5 wasn’t worth it. I mostly don’t “feel” the effects of 20 mpg until it gets above $3/3.50.