WD40 will damage rubber. At the moment the problem may be nothing more than the auxiliary drive belt needing tensioning. Spray WD40 on it & you'll probably need a new belt as well.
Edit: the drive belt is most likely some such material as neoprene, rather than rubber, which may well not be attacked by solvents. However, just spraying WD40 around isn't, I think, a good idea - better to find the problem than mask it!
I have read reports on WD40 being a water dispersant and destroying rubber yet myself and anybody I speak to has ever witnessed it.
Indeed WD40 manufacturers say it is OK. WD-40 Frequently Asked Questions
Originally Posted by WD40
What surfaces or materials are OK to use WD-40 Multi-Use Product on?
WD-40 Multi-Use Product can be used on just about everything. It is safe to use on metal, rubber, wood and plastic. It can also be applied to painted metal surfaces without harming the paint. Polycarbonate and clear polystyrene plastic are among the few surfaces on which to avoid using a petroleum-based product like WD-40 Multi-Use Product.
It is not a means to mask the problem at all but to exclude a loose/worn belt from the problem. i.e if the squeak stops then you know it is a loose/worn belt. It won't last and would soon start squeaking again so is never a permanent fix.
Lastly I would personally always
replace all auxiliary drive belts when replacing the timing belt, that is more important than being concerned about a bit of water dispersant on the rubber so they would be in very good condition to accept some WD40.
Just my humble opinion though.
However I'm happy to advise talcum powder if you have it!