I also have the 150.2 and have not experienced the problem you describe. So, to answer your question it is not normal for digital amp, at least not the 150.2.
My question is has the noise been there every since you had the 150.2? If not, then it probably is the amp. Also, if the noise is there only when the amp is powered up, then again it is probably the amp.
I like Palewin leave my 150.2 on 24/7 and when everything else is off, dead silence.
Just as an aside, it might be useful to also check your preamp as well to be 100% sure.
My question is has the noise been there every since you had the 150.2? If not, then it probably is the amp. Also, if the noise is there only when the amp is powered up, then again it is probably the amp.
I like Palewin leave my 150.2 on 24/7 and when everything else is off, dead silence.
Just as an aside, it might be useful to also check your preamp as well to be 100% sure.