Microwave's normal humming sound changing to a higher pitch?

[jenom]

Have a 5 year old microwave, working & heating OK.

However, now it started doing a random humming noise change.

The normal deep humming sound (transformer+magnetron) sometimes randomly changes into a higher pitched sound during heating.

Not sure, what is causing it, but it is an indication, that something is not working as supposed to be.

Any suggestion ?

Thanks!