Group,

There is a large resistor behind your head light that controls the fan
speeds.  For instanst, at 95 degrees C the low fan speed comes on.  Then
at 105 degrees C the high speed fan comes on.  What is happening is the
resistor is blown out on low speed.  So the fan is not turning on at 95C
and the temperature builds up til 105C, then the fan comes on. Causing
the
rumble noise you hear.

I don't remember what side the resistor is on, but its about 3/4" in
diameter and about 5" inches long.  It's a iron core resistor which
means it has a hole going through the center and is internally wrapped
with a iron wire.  The resistor "pack" sells for about $75 from a VW
dealer.  It is a resistor with a small wiring harness. All you need is
the resistor.  I suggest to find one used in a VW yard for about $5.00.

Any further questions, please feel free to email me.

Floyd Truskot