That's a good point which I had forgotten about. I had removed the thermostat at the time I did mine because I was replacing it.

But I like your idea about warming it up enough to open the stat. Sounds like it would work .....

Re. the moving parts. Yes the water pump impeller is the only moving part I can think of as well. I guess any grit/dirt present through age or corrosion in the system may also be in it. Possibly the coolant hadn't been changed in a long time before the last change so it may be flushing itself out.

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2001 X5 4.4i