Background noise

Hello folks. I have a Shindo Monbrison, a wonderful preamplifier, with a wonderful phono section. That said, I noticed a fairly high level of background noise when I turn up the volume without playing music (and this doesn't change by unplugging the interconnects). When the volume control is at its maximum level, you even listen to a little whistle.

Is it normal for a tube preamp or do you think there is a tube problem or another issue? I never experienced this problem in the past, but I've always used SS phono sections.

thanks.