So, about a week ago I noticed that the engine sounded a bit different than it usually does. It sounded "wheezy" if you can understand what I mean, like an old worn out Chevy V8 or something. It was really only pronounced at 3/4-full throttle. Now, since my #4 injector return line is leaky I thought maybe it was making this noise due to air intrusion after sitting and bleeding down. After another week, I noticed a new, very concerning noise. It was a rattly, light grinding, vibrating noise. When I shut the engine off it sounded like a marble being shaken in a soup can. Initially my mind went to the injector. However, the noise was still there even when coasting down in speed in gear (when the injection pump isn't injecting any fuel at all). I thought maybe it was a flex disc, accessory bearing, or vacuum pump. I had to drive it for a few more days before I was able to start looking at things. Easiest thing first, I took off the serpentine belt and felt all the pulley bearings. The water pump felt kinda funky and made noise, as did the alternator. I let the engine cool down to 40C and I took it for a quick run 1/10 mile down the street and back without the belt on. Still made the noise.
Seeing as how the noise was very dependant on engine speed and load (2000-3500 rpm was when it was loudest) I moved on and took the vacuum pump off. I had the VP off less than 2000 miles ago when I fixed my IP to engine block oring oil leak. It's the old style open bearing. I got it off and it looked fine. No issue whatsoever, smooth drive cam, smooth bearing surface, cage intact, balls in place. Next I took out the timing chain tensioner and reprimed it and reinstalled thinking that perhaps it was defective (brand new less than 7500 miles on it) and lost its hydraulic prime allowing the timing chain to rattle around. I replaced the thing in the first place because my old one did exactly that at idle when hot, and it bugged me a lot. I put everything back together, started it up and shut it off (no driving) and it still made that marble in a can sound. >: ( I chatted with my friend at this point telling him what I was doing. He reminded me of how tricky exhaust leaks can be. Then the lightbulb lit up. I did remember noticing that it sounded like it was coming from the rear of the engine or the trans area when I was driving next to an island on my left. Sure enough, my passenger side exhaust doughnut was gone leaving only the driver side one holding the crooked muffler, and the coupling in the middle of the car was wrenched loose allowing the muffler to rotate the entire back section of exhaust pipe. Now the wheezy sound made sense, and how it was way worse under full load, but still present when coasting. I tightened the coupler up and all 3 noisy are gone, wheezy, rattly, and Mr. Marble.
Anyway, I just thought you guys might get a laugh out of my wild goose chase. I'm just glad I found out what it was. I was feeling sick driving around listening to it thinking "My vacuum pump is gonna Glock itself any second now."