My most recent project-within-a-project has been to overhaul the cooling system in the X5. It has a new radiator, water pump, thermostat, expansion tank, expansion tank cap, upper/lower radiator hoses, and fresh BMW coolant.

I was having a problem with it running hot, but have not gotten it out on the road yet to see if the cooling system overhaul did the trick. The last time I drove it, I noticed a strange sound but thought it was a bad idler pulley on the serpentine belt. I replaced the pulley when I did the cooling overhaul, but noticed the sound was still there so I listened a little closer.

I have my suspicion of what it might be, but I want to get some other input before I go there. I shot a quick video with my phone and the noise can be heard every time I rev the engine. I'll quickly rev it to 2,000-2,500 RPM and you can hear the "FFT-FFT-FFT" sound each time? Any theories about what it might be?

I tried to embed a video I uploaded to YouTube, but can't get it to work. So here's a direct link: https://www.youtube.com/watch?v=v_5gQpTYfzE