harmonic squealing sound from behind dash
 

When I'm on the highway and I get up to around 65 - 75, I occasionally get this really odd harmonic squealing sound that comes from behind the dash on the drivers side. I've tried to see if it happens at certain speeds or conditions but can't really put my finger on it. Sometimes I'll go for hours and not hear it and other times it will go for a few seconds to a few minutes. Sometimes really loud, other times not so loud.

Anyway, it's going in tomorrow for service B and I mentioned it to them, they said they would look at it but if its something that is common or easy, I'd rather do it myself and save a billion dollars for something else.

Anyone got any ideas what it could be?