The windows are most likely rattling because the felt strip around the edges has disintegrated. Mine were the same. I replaced the channel with new felt from here:
http://www.woolies-trim.co.uk/
It's not the easiest stuff to fit. I found the best way was to gradually work it around the window with a screwdriver while sliding the window back and forth. This way you don't have to disassemble the window frame. It works great though without resorting to bodges, and no more rattly windows
Steps:
1. Remove the window lock so that you can slide back and forth without snagging.
2. Using a flat head driver scrape out old felt from top and side channels - Note make sure you gently pull the felt out from under the part of the channel that is never exposed, you don't want to leave that in the way for later step.
3. Using flat head and pliers remove bits of plastic from bottom channel (not necessary if your bottom plastic runners are still in good shape) - Note make sure you gently pull the hard plastic out from under the window, don't break it off until it is clear.
4. leave your new felt/rubber channel in one long role, start at the top by slowly feeding it from the open side to the fixed side as you move the window back and forth push it along above the window until you can grab the tag end with pliers and pull to the rubber window stop. (do not glue in place).
5. then work it around in one long piece until you get to the bottom side of the sliding window.
6. measure and cut the remaining length for what you will need to complete the job.
7. again work it under the slider until you can pull and run to stop.
8. Then pull both corners out a bit and trim the sides where the bend forced it to bunch up so that the glass can seat properly.
9. Glue in place if you wish. Best to leave unglued for a while to make sure all is well.
The lower channel should be able to drain rain water away.