OK. Next guess. [Linked Image]

I still believe it is a physical phenomena.

How about a crud burst? Perhaps as the fans change speed, they produce a resonant frequency that shakes loose deposits that are in a low flow region?

Has there been any changes in the water system that would have released plated out deposits? Any recent pipe replacement, changes in water chemistry, physical work on the cooling towers, additional loads added to the system, etc.?

LarryC