Recently I noticed that the heater fan in my C4 Grand Picasso was becoming very noisy, sounding similsr to the engine being revved at 2000 rpm plus. As I had to dismantle the lower dash panels anyway to replace the blocked heater matrix,( blowing cold on the drivers side) I took the opportunity to check the fan which is on the drivers side.No problem found with the fan, but the pollen filter was badly mis shapen and distorted.(After replacing this, I need new knuckles and a blood pressure check.I will not bother to replace it it again.Its like trying to push a piece of flat carpet uphill and through a slot that you can't see, using only your grazed fingers!)
Anyway, the noisy fan is fixed, I guess the mis shapen pollen filter was causing restricted and noisy airflow, possibly vibrating.