98 397 strange humming sound coming from the engine copartment

I can hear this at idle. it's a higher frequency hum coming from somewhere under the hood. That's about all I can narrow it down to. It's not belts....not a squeal, but a hum.

Could it be power steering pump?
Fan clutch?

Any ideas will be greatly appreciated.

Motor is a 3406e 6TS

Thanks guys,
Toby

 

It's been doing this for a very long time. Can't remember when it started. May have been doing this when I bought the truck 4 years ago.

Anyway.....I hate to run my truck when I sleep. So, most of the time it's off, unless it's warm out and I need to run the A/C.....or, warm up the truck for heat.

It's really getting on my nerves when trying to sleep. I'm sure it bothers they folks that park next to me too.