Loose window pane

Bedroom window glass pane is ever so slightly loose. I wouldn’t have noticed it but with the wind I can hear howling and the glass rattling.
Whats the best way to fix it?
The windows are standard double glazed UPVC, although they are decades old. The glass seems to be held in by a rubber seal. This seems to be missing along the top which could be the issue. I cant find the exact seal anywhere for a like for like replacement. Can I fill the gap using silicone sealant or glazing putty?

Thanks

Buy a gasket (seal) sample pack, decipher which seal best fits the gap and then buy a longer length of that type.

Double Glazing Seal / Gasket Sample Pack (handlesandhinges.co.uk)

Other sites selling these are available, that's simply the first one I found.

Thanks, will the seal need to be held in by silicone? I’m guessing over time the last became loose and fell off?

Sfrazi said:

Thanks, will the seal need to be held in by silicone? I’m guessing over time the last became loose and fell off?

Click to expand...

No, the gasket should be a relatively tight fit, thereby holding itself in the cavity. Don't use anything like silicon, not needed.