Squeaking in certain gear combos

Jockey wheels is my guess. I get this when they have got wet and washed oil out and then dry and they sqeill. The reason for I am guessing it happens with the gear combination is the faster chain travel (just a guess). Anyway I remove both jockey wheels clean them, relube and reinstall them and it sorts it. 

Weird B-screw adjustment?

If by chance you have chainstays with 405mm lenght not every chain brand will work OK.

I have a giant tcr advanced sl and a grinding noise appeared as described when I replaced the original kmc chain by a shimano chain. I've then putted a kmc chain again and the noise went away.

Cheers,

Filipe Dias

Sounds pretty similar to what I have experienced - I can especially relate to this "chirruping" sound.

I took my jockey wheels off and they had pretty much gunk stuck to the sides of the wheels. Since the top jockey wheel needs to be able to move sideways (on shimano at least) then I believe the stuck gunk prevented that movement from happening freely. Whether this caused the sounds or they were just worn out, I don't know. 

Instead of cleaning the wheels, I just replaced them as they had already seen some 12000km. The chirruping sound has been gone since.

Your issue might be different but I thought that I would share my experience anyway.

If it's a high pitched squealing/ chirruping I'd be going for jockey wheels. Chain under extra tension/ particular angle causing rubbing on some part (perhaps the squared washers they fit in?). To eliminate it I'd try a spray of wd40 around the jockeys and their bushes.

If it is fine on the smaller chain wheel across all sprockets, then not sure the angle is the issue.
Big to big might be worth checking the position of the rear cage.

You're describing a cross-chaining scenario. I had a similar effect after I'd cleaned a cassette - totally stripped it of all oil and residue - and it was the inner plates of the chain rubbing against the cassette teeth as the angle of the chain increased. A temporary squirt of WD40 on the cassette silenced it, and then giving the chain a (probably excessive) coat of lube fixed it for good.

Take off and nuke it from orbit - it's the only way to be sure.

Rear derailleur sounds a good bet.

Remove seatpost, clean it, the tube and the fastener, lube everything and put it back together.

This has stopped what I had always assumed was a creak in the bb or chainset.

And it's never a bad idea, chatting to a bloke unlocking an Orange mtb ouside the supermarket the other day, lovely bike, there was such disappointment in his voice about the seatpost cold welding itself into the frame.  LBS could do nothing.  It reminded me, and I will be doing this to my everyday bike this long weekend.

That the noise only happens with big/big cogs, then I'd be thinking about chain tension and rear derailleur (as the front derailleur has already been checked). Is the chain long enough and are the jockey wheels in need of replacement?

Check chainring bolts are nice and tight.

First check for chain rub on the front derailleur. If it isn't that invest in one of these:

https://www.ebay.co.uk/p/Auto-Mechanics-Stethoscope-Car-Engine-Block-Dia...