A couple of months down the road, and I thought I'd bring some resolution to this thread. Man was I waaaay off! Thanks to Numark318i, who told me that it was the water pump on his M54 that was making the noise. Sure enough, after I replaced mine, the noise is now gone. I should have done more research tho . . Turns out it's the same identical water pump from the E36, and I had a spare one around from my old 325. But I didn't find this out till after I bought a new one
At least the job only took me less than an hour.