I have a 9 speed ultegra cassette and rear derailleur. In the smallest 4 cogs on the cassette the chain will not run straight on the lower jockey wheel of the rear derailleur. It comes in at an angle which causes a rattling noise. On any of the other cogs it is fine. While in the big ring there are NO issues either.
It's a new SRAM chain which I'm certain is the right length.
Anyone got any ideas as to what the problem is?

Thanks.

P.S. Apologies if this has been dealt with before but I have trawled the forum and cannot find a similar post.