Join Date: Aug 2012
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Noise when using the defroster
Bought my 2000 F350 SD XLT a month ago. One of the items I looked at on the test drive was to see if I could change the HVAC selector to different positions and if the blend doors would keep up, ergo to test the vacuum pump. All went well. So I moved on.
Drove the truck into work this morning. Weather was bit cooler, so I had the heater on, fan hi (4th position). All was well. Then I switched the mode selector to the defroster position. I was surprised to hear this noise coming from underneath the dash on the passenger side...it sounded like something was rolling around in the air box. I thought for a half a second that a blend door must be broken, but that didn't seem too likely. So I played around with it a bit. Found that only when in full defrost mode and only when the temperature selector is set to full-max-heat do I hear this noise. If you turn the temp selector to a cooler setting, the noise disappears. If you turn the mode selector to half-feet-half-defroster and have your temp selector to full-max-heat then you hear the noise but much less often. Any other mode settings, feet, face, combo thereof and any temp selection other than full heat then you don't hear the noise.
On the test drive of course the weather was warmer, so I only had the coolest temperature setting selected. Thus no idea if it was an existing problem or not. 'Seems likely but whichever. I'll add that to the list of stuff to check the next time out!
Anyway all of this had me scratching my head as I dodged traffic. Been too busy thus far to think about it much. Thought I'd post here and see: has anyone else encountered this? Any ideas? It would seem that the vacuum pump would be ok. But I dunno.