Stop hardboard floor from creaking?

My bedroom floor is creaking. I removed the carpet to reveal a hardboard covering. When I remove the hardboard panels I found that the floorboards themselves don't creak at all. My floorboards are a little uneven and I think I need to keep the hardboard before laying the carpet. I've searched the internet but can't find a solution. It would seem that hardboard shouldn't squeak. Can anybody suggest how I can fix the squeaking that the hardboard covering seems to be causing? Should I glue it down to the floorboards? Put more hardboard nails in? Seems there is ample nails already. Many thanks.