When it rains I hear an annoying drip drip from a windowcill. IT is made of
a hollow plastic extrusion and the drips really make a loud noise that
passes into the lounge.

I wondered about filling it with expanding foam - but:

Would the foam reach half way (from each end)?

Would it react badly with the uPVC?

Could it distort the uPVC as it expands?

Any ideas?