Dear Tom and Ray:

I did a front-end brake job on my '89 Dodge Ram 150 truck. I took the front
rotors off and had them turned at a local shop. I put in new pads and put
everything back together correctly. I drove away the next day and experienced a
slight vibration that had the sound of a low hum, like big, off-road tires on
pavement. It only occurs when applying the brakes firmly. I checked to be sure
everything is tightened correctly, but I still have the hum. What could be
causing this? -- Phil

RAY: What's causing this? The guy at the local shop that turned your rotors,
Phil.

TOM: He did a lousy job. He did, what we call in the trade, a "record cut,"
because the rotor looks like an old 33 rpm record, but with deeper grooves in
it.

RAY: Ideally, the rotors should be perfectly smooth. You put the rotor on the
lathe, and as the rotor spins, you move the blade from the inside to the
outside. But if you move the blade too quickly across the spinning rotor, cut
too deeply, or use a dull cutting tip, you'll end up with grooves. And when the
brake pads press up against the grooved rotor, you get that hum.

TOM: So you can either wait until the metallic particles in your brake pads wear
down the grooves and smooth them out, or you can go back and ask them to redo
the rotors -- assuming you have enough thickness left in the rotor to cut it
again. Good luck, Phil.