Strange squeal when dropping motor to idle

Well, I'm sure i won't be able to describe this accurately, but here goes;

Whenever I'm running the engine (rpm's don't matter, temp doesn't really matter) and drop it back down to full idle, the motor comes down to idle like it should, then i hear the strangest noise that is basically a whine that starts high and lowers in pitch, then goes away. Some are longer than others, but I have been back there trying to find the source and have been completely unable to.

For some back troubleshooting history that may be relevant, the last time this started happening coincided with a faulty water pump, and went away when the pump was replaced. It just recently started happening again, could I have possibly not put my impeller in correctly?


Any help is greatly appreciated, thanks in advance guys!