The drive belt may be slipping or going bad.
If it is a Toyota, they do not know how to fix the squeaking brakes either. Even after replacing all the pads and machining the rotors and rear drums
If the work was done recently, then the pads have not yet worn into the rotors. This can typically take up to 200kms to wear in properly.
buy good pads, and install and have rotors machined
This is probably due to worn brake rotors on the brakes, if there is no vibration when driving then this will be the cause.
Most IS 250s, depending on driving, get about 25000-30000 miles on the front brakes and rotors.
A common reason is that the front rotors have warped. Go check out WikiAnswers on the topic of "warped brake rotors". Rotors should wear out smoothly in normal driving. Use of the brakes that generate lots of heat (hard braking, riding the brakes) will result in warped rotors.
Warped rotors. Driving using the left foot to brake causes the rotors to heat up. Quenching the hot rotors by driving thru water will cause them to warp. The warped rotors cause the pedal to pulsate.
New brakes do not grind. If you replaced the brake pads and resurfaced the rotors you may hear a bit of a sound but it is normal until the pads and rotors wear during normal driving.
It depends on your driving style and the condition of the calipers. If you ride the brakes or otherwise drive in such a manner as to cause the brakes to overheat, the rotors will warp prematurely. If you are constantly in stop-and-go driving conditions you will wear out the brake pads and rotors much faster than someone who drives greater distances and seldom uses the brakes. I've seen some people who get little more than 20,000 miles from brakes and/or rotors. My last brake pads and rotors had over 70,000 miles when I changed them, but then, we drive long distances here.
If you brakes are still squeaking after you change your brake pads in any car, you have a warped rotor. The squeaking is caused by the rotor vibrating when pressure is applied by the pads. The best way to permanently fix the problem is to replace your pads and your rotors at the same time. This can also cause accelerated wear on the new pads if you do not replace the rotors at the same time, or at least have them turned.
the brake pads are probably worn - there are wear indicators in the pads made of solid metal that scrape on the brake rotors when they are spinning. When you squeeze the pads by applying the brakes, the main part of the pads make contact and muffle the sound. pads are cheap for most vehicles and easy to replace.
There was a lot of squeaking while braking when my car needed new rotors.