65 C10 250 stock..high frequency pure tone??

[sharkskinman]

Runs as it should.. no other issues that haven't or will be addressed

It just started
The tone doesn't change with RPMs
If I pull throttle down to 5-600 it goes away
But just start and hear it

Does it regardless of sitting at idle, driving or shifting..etc..
Does not change pitch with movement of clutch or higher RPMs

??? Can't think of anything that would make the sound and Not change at all really with Any changing of anything

[geezer#99]

Vacuum leak.

[sharkskinman]

Too pronounced top be a vacuum leak
but I'll check

Thought water pump (radiator leak so Only water n it's getting rusty color
But also have a noise sounds like a marble or two when I start till good n warm then every once in a while