Maybe it's pinging. That would more than likely increase with throttle load (especially at the lower RPMs), and engine temperature. You might want to check out the knock sensor for proper operation/connection. Also, could your ignition timing have been too far advanced? Or the cam out of time, taking the ignition timing with it? When you reassemble it, you can verify all that. Keep in mind that if your car has the damper pulley with the rubber isolation, it may have slipped, in which case the timing marks on the belt cover would be useless and you would need to go by the mark on the crank timing belt sprocket.

If you car has OBD, it might indicate whether or not the knock sensor is doing its thing.

Maybe I'm barking up the wrong tree with all this babble, but it is food for thought.