My 2009 GMC Sierra 6 cyl. auto, 2wd, 2 door, 8' box gives a steady squeal at the instrument panel which will sometimes disappear then reappear as I'm driving. This squeal has become more frequent with the colder weather & has become so loud that I can't listen to the radio. This usually occurs between 75 to 100 Km/Hr. I shifted into neutral & took my foot off the gas pedal but the noise remained until the truck slowed down to about 60 Km so I don't think it's related to the engine RPM but more to the power train or maybe wind sourced. There was little variation in the pitch of the Squeal ; only the volume varied & it didn't appear to be related to the RPM's. Anyone with an idea that could trace the source?

 

I don't know if you have a speedometer cable, but if you do and the cable has gone dry, that might cause some noise.

 

Thanks Kazoocruiser,
I checked & apparently I have a speed sensor at the transmission which feeds an electric signal to the speedometer & does not use the mechanical speedometer cable of past vehicles. From searching other threads, I found that my case doesn't appear to be an isolated situation as there are several reports of noise from the dash/firewall area that others have experienced with a mix of root causes & solutions. I'll have to eliminate these one by one to track the source. I don't want to remove the dash trim board panels to access the source unless absolutely necessary but unless this thread works out I might be forced to go that route.

UPDATE: Today i was increasing speed with the fan on low setting and at 70km/hr heard the squeal starting. I then turned the fan up to full speed which resulted in a much louder squeal while holding my speed steady. I then cracked the side window slightly & the squeal stopped then I closed the window which resulted in the squeal returning then I shut the fan off & the squeal stopped. So now I have to locate the exact source (door seals or firewall leaks which appear to be the result of wind /air flow producing a squeal sounding whistle. It seems that with higher driving speed or high fan setting the cab is being pressurized & the increased pressure is contributing to the whistling.as air is escaping from the cab through the as yet undetermined whistle source.