Is it normal to get a squeal from the speakers when changing chanels or when signal strength makes the STB loose audio?

I have a Nintaus DT-200 and the squeal can be very loud, depending on the current volume setting.

No, not when changing channels, but yes, when signal strength drops too far (depending on how smart the software in the STB is)

It shouldn't do that, but it does happen with a few models of STB's. It usually means they haven't set enough sound blanking in the software to cut the sound altogether instead of hearing the loud distorted squeaks, scratches and pops. The Teac DV-B420 makes the same noise you are talking about whenever I change to 7 or SBS. Due to our complaints here, Teac are fixing this problem (hopefully by introducing longer sound blanking rather than just muting for a split second when changing channels) in the next software upgrade.

Is the Nintaus software upgradable? If it is, make sure you have the latest software, and if that's already the case then nag the hell out of them to upgrade it again