Same on mine.
Mine needs the right input signal to cause the noise though. I found the sound is very signal dependent. Different signals change the pitch, make it get louder, quieter.
Does yours do it with no input signal? Any impact of enabling other channels?
Mine seems to happen when I'm looking at high frequencies. Could be because that's when the finer time-base settings are used, like you mentioned.
I haven't dug any deeper to investigate why.