Sure it's not a slight air leak from something disturbed ?
As for belt tightness the basic twist thru 90 degrees is fine, they don't need to be mega tight.
If you use a long screwdriver put onto where you think the noise is coming from and the other end resting on your ear you might be able to better point out where it's coming from.
One thing I have learnt with cars is sounds can come from some very odd places but sound like it's totally somewhere else, just a thought.
tabetha
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