A bit of an odd one this. I'm not even sure if it belongs under this category, so Administrators feel free to move it! I checked a load of the existing posts for something relevant in this category and wheels and tyres, but I couldn't find anything. Hopefully one of you clever chaps will be able to help me out a bit. I'm usually pretty good but this has got me stumped!! I was driving my '99 CLK320 to work this morning and got stuck in traffic. I noticed that there seemed to be a knocking sensation coming through the pedal every second or so. I pumped the pedal, stuck it in neutral, then park, but nothing made any difference. I thought I might be able to hear it so I turned off the radio, but still couldn't make it out over the noise of the heater fan. I turned the fan off and, to my considerable confusion, the noise/sensation stopped! On returning from work (still happening on the way home when the heater's on) I had a chance to take a closer look. It appears that the knocking occurs roughly once a second when the engine is running (in any gear) and the fan is on (any speed). On opening the bonnet, the knocking can be felt when touching the brake servo even when no pressure is being applied to the pedal. Any thoughts/help would be gratefully received. I cannot think of any reasons why the braking system would be linked to the fan.