Creaky rear wheel when peddling

[saladfunky]

I have developed a creaky rear wheel creaking and cracking when peddling even at slight pressure although it is a little intermittent being worse after riding for a while. Sometimes it is ok until I peddle firmly and then it will creak at any pressure. Sometimes if I freewheel it stops for a for peddles. Last night it stopped a couple of times when I changed gear. No creaks when freewheeling. I know it is the wheel as I tried a different wheel last night and it was fine. Could it be a worn freehub or cassette? Or spokes maybe? I tried grease where the spokes cross. Everything outside is lubricated with gt85. It has done about 2000 miles and the chain is only very slightly stretched.

Anyone any ideas or similar experiences?

thanks

Chris