Vacuum Pump Test: How to Check & Repair

How can you test the vacuum pump on a TDI engine?
Is this even possible? The pump is supplying pressure to the BKV.
"I'm now experiencing a very loud humming or rattling noise when the car is stationary. However, this noise only occurs when the brake pedal is pressed! Based on this, I suspect it might be the vacuum pump. The workshop mentioned that the gears might have play on the intermediate shaft, which could be causing the rattling. But could it also simply be the pump itself? I'd like to have it checked, especially to see if there's still enough vacuum pressure for the brake booster."

Thank you.

Has anyone ever had a problem with the vacuum or vacuum pump?

So far, the problems have been limited to the O-ring for the Zyko.

CU Gremlin.