About 2 to 3 weeks ago the car has started grumbling when reving the car up with and with out a load on the motor. I have a 02 wrx wagon with 93000 miles on the stock timing belt. It's not bad on idle but changes with engine speed. I changed the oil and filter and no metal shaving in the oil. I took off the accessory belts to make sure it wasn't the accessorys. It doesn't use any oil and is clean?