Grinding noise coming from the front of the car

I recently purchased a used the car listed above (SES model). I also recently found out it has a rebuilt transmission. I did a Carfax search on it. Was given a warranty from the dealer.
After a few days of driving it, I noticed a grinding noise coming from the front of the car. Sounds like a fan rubbing on something. Does not make the noise everyday, or all day. It will make it when I step on the gas pedal in parked or while driving. I had taken it to a mechanic, however it was not making the noise at that moment. He gave the car a once over and said all looked fine. I looked online and found many different possibilities. Ranging from brakes, to tires, to transmission. I have now on top of this hear a faint knocking from the engine.
Hopefully both nothing on the serious side.
Thanks