When I worked in avionics I chased a lot of noise. The good news is that 80%
of the problems were antenna grounds; the bad news is that a couple of cases
defied all my efforts. I recall a French Socata that had magneto noise so
bad I could hear it when it was flying nearby! I never figured that one out.

The normal reason for interference with AM reception is indeed grounding.
The most critical area is the coax ground where the antenna mounts. Remove
the antenna and scour the area of the body down to shiny metal where the
metal part of the mount contacts. 120 grit sandpaper works, but my favorite
abrasive for that is Scotchbrite. Remount the antenna with dielectric
grease, silicone grease or even petroleum jelly and see what you have. You
may also have to finagle with the hood ground, even to the extent of
fastening a wire between the hood side and the body side of one or both
hinges - but if another car parked along side yours doesn't hear the whine
from your engine the hood ground won't help anyway.

If you still have a lot of whine things get a lot tougher. The next step is
to make sure the noise is coming in the antenna by unplugging the antenna
from the radio and seeing if the whine goes away. If it does, you will have
to do some detailed investigation of the shield ground on the antenna coax.
It should not contact ground except where it was designed to - at the
antenna mount unless there is a filter, amplifier or other device hidden
along the way. If it does not, the problem is in the electrical system. (I
suspect you are hearing ignition rather than alternator; ignition noise is
very raspy while alternator whine is fairly smooth.)

For the electrical system, start with the easy stuff first again. Clean the
battery connections and look at the alternator connections. That probably
won't help but you'd hate to overlook it and have it be critical to getting
the noise out. Next step: measure the AC voltage across the battery with a
digital meter - it should be less than 0.1 VAC. If half a volt or more the
alternator almost certainly has a bad diode.

Still noisy? (Whew!) You may be dealing with a ground loop. These nasty
devils are the result of current going through a ground path that includes
the ground path of the receiver. The common resistance couples the noise
into the receiver. At that point you might just want to swear off listening
to AM, but if you get to this point check back and we'll work through your
options together.

Mike