Thanks for the replies. I didn't really mean to climb under the car when it's running and in gear, more looking from the side/mirror idea to see if there's anything obvious.

I should have stated this is an 85 w123 turbo that I just converted to manual, and deleted the rear muffler with a custom exhaust. I work as a machinist/welder so it's not just a hack job, everything seemed solid. The car seems to drive and sound great other than this weird noise around 2k rpm in 4th gear. The noise could be a resonance thing, it's coming from the rear of the car. I'll do some investigating and report back.