Thats an easy one. I am the broadcasting field. The AM band is a very low frequency. Measured in kHz. The earth produces magnetic waves that travel in this frequency band. So does most electrical equipment. To pick up your engine noise is normal. Its all the electronics involved that are sending out the wave and the antenna is picking it up. AM radio waves follow the contour of the earth during the day, going around buildings and such, not like FM which is line of sight. AM waves also bounce off the ionosphere at night. Causing the signal to travel even further. I live in Oklahoma and I have a buddy that worked at a AM station in Okla City. He used to get callers from GUAM at night. Quite a distance for that signal to travel. So to answer your question. Yes, it is perfectly normal to pick up engine noise on the AM band. Like if you passed under a set of HIGH voltage electricty lines, you would hear the hum on the dial. If you live near the station you are trying to pick up on the AM dial, its signal should be strong enough to power over the engine hum.