If you are willing to upgrade the shifter as well, one of the nicer Shimano mountain bike derailers, such as Deore, would be a good upgrade. If you want to keep the shifter you have now, Alivio would be a good buy.
I suspect the derailer pulley sprockets are just worn out. In order to verify this, spray some WD40 into them. It isn't a permanent fix, but it will make the bike quieter if there is a problem. So lubricate them, and if the noise goes away, you know they are to blame. A set of new pulley sprockets probably costs as much as a new Alivio derailer.