Odd chewing noise then a vacuum plugging noise on startup

[bigtoe]

Hello all
I have a strange intermittent problem with my 2008 d4d. Occasionally on start-up I get a strange chewing sort of noise from the engine with a squeal. It sounds like a belt slipping on a pulley. Then within about 10 seconds it seems to disappear. Just before the noise disappears you can clearly hear what sound like a vacuum being generated more and more until it sounds like it gets plugged and then all is well. Definite lack of power when the noise is present. My first thought was the vacuum pump pulley is slipping but looking further into it I don't think the vac pump is driven by the belt. Is this correct.l?Any ideas on what might be going on? Cheers everyone for your help and thoughts.

[bigtoe]

Ok just incase anyone ends up with a similar problem it turned out to be the alternator pulley slipping. Once the noise hung around long enough for me to do some test it was easy to find.