Have you checked both your front brake calipers for release? I suggest that you lift the front end off the ground, spin the front wheel and see if any noise can be heard. Remember to apply the front brakes prior to spinning the wheel. You may hear the scrap of brake pads against the rotors. If one of the caliper are not releasing, you should be able to hear the knock as it releases. Normally lube required on the sliding pins. Oh and if you ask how to lift the front wheel off the ground... make use of the search function, there are many threads on the subject.
If you think the sound is from the left engine cover, then by all means check the magnets. Normally your ear against a plank or screw driver that is against the cover will let you localize the sound. For the magnets take voltage readings at different rpms. Let us know if you find anything amiss.
Last edited by Gert; 08-25-2018 at 03:19 AM.