'98 C230, 64K miles... HVAC vacuum distribution block started making a shrill noise... intermittently at first, progressing to full time a week later. Noise ceased when power to the block was cut. Installed a new block, which remained quiet for a couple of days and then started making noise. Installed yet another block, which is now also making noise, but only intermittently.

Any ideas as to whats causing this? Are the new blocks just faulty?