The 1967-1972 – What’s That Noise? Gaining speed after you turn onto the highway, your GM truck (1967-1972), moves toward a cruising speed equal to the surrounding traffic. As your engine reaches about 2,000 rpm you suddenly hear a low hum up front. It does not stop as the truck speed increases. If you lower the windows, play the radio, or turn up the fan blower, this hum is not so noticeable but it is still there. How will you locate this noise source when the truck is stopped?
No problem. Others have researched this mystery noise, discovered the source, and stopped it. Who would have thought the culprit is the hood springs? It appears that on many GM trucks of this body style, the two coil hood springs develop this hum (like a tuning fork) as surrounding air speed increases. The sound becomes magnified as it transfers to the large sheet metal hood.
This noise is easily stopped by filling the coils of the hood springs with a towel or carved piece of foam. To produce what a difference this makes, tap your hood spring with a hand tool and listen to the echo. It does not occur when the coil is filled with material.
Who said automotive engineers walk on water?