Bicycle Mechanics - Braking noise

The front brakes on my new Trek 1000 make some aweful noise (like sandpaper scratching hard on metal) while braking. The rear brakes are pleasantly quiet. What might be the cause? I'm going to take the bike in for a free post-purchase tune-up soon but just want to know if you guys have any advice.

Inspect the pads to see if grit has gotten embedded in them.

most likely, you have worn out your brake pads and are down to the fastening pieces that hold the rubber pad to the brake mounting fixture.

If you remove your brake pads, you may see what looks like tiny aluminum grit imbedded in your pads. Time to change your pads.

Not all brake pads are made alike. Some are super grippy, but only last for a very short while ( I have heard of some designed to last for only a couple days of use).

If your use does not demand standing on the edge of a rock cliff, you could probably stand for more durable, though less grip-power pads.

I just bought the bike last week so it's unlikely that the pads are all gone. In fact they look pretty new. I checked for grit but found none. Could it be that the wheel is untrue? How do you check that?

Hmm... I dunno. Sounds weird.

an untrue rim can cause a lot of problems, but a grinding sound when braking wouldn't be one of them.

Is there a chance that the brake pads are set too high on the rim and you are coming in contact with the tire sidewall?

You could try sanding the braking surfaces down with steel wool. Trek has been spec'ing their more basic models a bit lower lately, and you may have an inferior front rim. Is the braking surface smoothly machined all the way around?? I would bet that it isn't. If not, you could take it back to the shop and complain. If enough people do, then maybe Trek will pay better attention to their supplier quality in the future.

Check that the adjustment of the pad to the rim is perfect. If this is beyond your ability, learn how to do it.

Your brakes are one thing that must never fail.