Weird Noise Developing At Speeds Higher Than 35mph?

[camshaft]

Background: 05 Sierra 1/2 ton crew 4x4. 85000 kilometers on the clock (52000 miles). Truck babied since new.

 

As recent as a few weeks ago I started to hear what sounded like a noice soming from behind the dash/firewall area once my speed reached about 60km/hr (37mph). The noise is barely noticeable with the radio off and is constant from that speed on up. Almost appears to get a wee bit louder as speeds rise.

 

Kinda sounds like a noisy lifter, but when I put the truck in neutral and gas it, there is no change either with load or without. I'm satisfied its not engine related. I thought maybe wind noise as I drove faster, but checked the obvious things such as mudflaps, even took off the bug deflector, checked the weatherstripping, looked for anything that might be loose (roof antenna ect) and nothing seems out of place.

 

The best description I can give is it almost sounds like "gurgling water" when I get up to speed. I thought maybe something to do with the heating/ac controls. I have tried every setting, turned it off ect and no change.

 

Anyone experience a similar noise? I'm hoping its not something like a bearing. I have greased everything routinely and that seems like pretty low mileage for a bearing problem.

[txab]

Have you checked your coolant level for proper fill?

[camshaft]

I'll check, but from what I've read most coolant issues are on start up and only last for a few minutes then go away. At least in terms of the "gurgling" noise.

Mine begins immediately once I hit speed whether engine has warmed up or not and does not go away.