B screw tension not right, so jockeys rubbing.

Worn top cogs.

Chain worn at different rate to cassette

Loose cassette

The cassette is tight
I,d say I never really use those high gears as this is my cx race bike.
Will check b tension.
The chain could be old. But why only the noise when in these high gears?

You kinda answered your own question.
I get the same noise when using sprockets on my cassette which rarely get used, those sprockets are basically new, while your chain is not and worn to the other sprockets, if you get my meaning?!

so would a new chain do the trick - might not like the cassette though ?

Chain dragging on front mech, due to the increased angle to reach the cogs?

If you're calling "high gears" the smallest sprockets, my guess is indexing or a lose cassette.

If you mean the largest sprockets, then it's likely b-tension. The derailleur will push against them if the tension is too low, and the teeth on the cassette will cause it to vibrate like it's being dragged across a washboard. This usually results in a rumble sound/feeling.

yep high gears the smallest sprockets.

cassettes nice and tight.

front mech isnt rubbing.

It's going to be uneven wear, based on the information you've now elicited.

A new chain may be fine, but most likely it'd make sense to go for new chain and cassette, and keep an eye on the chainrings for chainsuck.

Bent mech hanger?

Pictures!