I can't offer a specific solution, only things to check out.
Take the roof-top cover off the unit and do a quick inspection of everything. Make sure a critter didn't move in, or a foreign object didn't get sucked up into it causing strange noises. Also check the mounting bracket for the cooling fan up there. My brother's a/c unit on top of his Starflyte, the cooling fan motor's mounting bracket became loose creating an odd sound. He went a step further and fabricated an extra bracket for additional support for the fan up top to assure it wouldn't happen again.
Also tighten the screws that hold down the a/c unit. They might have worked themselves loose, creating unusual sounds.
While you have the top cover off, make sure the cooling fins are clean of debris for maximum cooling performance.
Good luck!