I have a 1999 4 door 4WD Tahoe that has the most annoying noise (kind of a cross between a rattle and a squeak) that seems to come from the dash area. It is very inconsistent (occurs about 75% of the time) and is only at highway speeds, never around town. Does not have to do with bumps or rough roads as it does it on smooth interstates. I have practically taken the entire dash apart and can't find a cause. I have even checked the outside mirrors and one of them was a little loose but not the problem. I have seen threads about steering column but this occurs while driving straight on smooth roads so don't think that is it. I have been searching for this noise for a year with no luck, any suggestions will be greatly appreciated