New M42 whine - would love some opinions on what it could be

[jsnppp]

I've ordered a new AC belt idler and will swap alternator + check belt tension this weekend. Fingers crossed!

Daughter reports driving it today and the sound didn't appear.......

[jsnppp]

Thanks Toby - I'm pretty sure that I installed new idler when I did the profile gasket but new doesn't mean much perhaps. It's still possible its chain tensioner too... I'll look at my receipts.
...

[jsnppp]

Cool, thank you - belts first for sure just to rule out stupid easy fixes. Would be great!

I was reading maybe chain tensioner but for whatever reason this doesn't feel like that, especially...

[jsnppp]

Curious what you all think of this whine from my daughter's M42. This is a new sound that appears when it warms up a bit. Changes with RPM, not motion based. Doesn't change with moving the steering wheel....