I've turned a few wrenches in my time. This could be anything, worn or lose asseccory belts, low oil pressure/low oil, exhaust leaks etc. It's hard to tell without being there in person but I would start with the broken exhaust bolts since this is a known possible culprit and probably an easy fix. I would still check your belts too, my mom pulled up to the house one day complaining about a similar noise issue. I poped the hood and to my amazement, her belt was only held together by a handful of threads. The broken/tethered/worn pieces was slapping everything as the engine turned.