'90 535i : Thermostat Replacement leads to rattle?

[hdaemon]

I just finished doing a coolant swap and replacing the thermostat in my '90 535. Ended up being a bit more of a job than I had hoped, as one of the bolts in the thermostat housing decided it wanted to shear off rather than come out. So significantly more coolant hoses were removed and disassembly done than a normal thermostat job.

In any case, while waiting for the car to come up to temp, I started getting a fast rattling noise from the front of the engine. Couldn't tell exactly where it was coming from, but when I cracked the bleed screw and let out some air, it went away... for a bit. Every time I would crack the bleed screw (no bubbles in the coolant I could see), it would go away, but then return within a few seconds of my closing the bleed screw.

Any suggestions? Still air in the system that I need to bleed out?

[Rus]

Water pump perhaps? Cavitation can cause noise in pumps, but I'm not sure if its something that can occur in an automotive water pump. Did you try to get as much air as you can out of the system before start up? I usually have to burp the upper hose a few times while playing with the bleeder to get as much coolant in as possible before first start.

[Derek A.]

ever since I did a pump on my car I get intermittent noise. Sounds like the pump is turning into powder. after the car warms up and the revs are up for a minute its really quiet. One of these days I will swing it by the indy and let them take a listen.

[Ross]

I'll bet a dollar to a doughnut the v belts are the cause. V belts can make some horriffic noises when they chatter. Coolant was probably spilled on them causing this. There where lots of water pumps sold to unknowing folks back in the days when all cars used v belts.
To check my theory spray a little water on the belts, if the noise stops you've found it. Clean and properly tighten the belts, or better yet replace them with new ones.