AM has been and always will be susceptible to electromagnetic interference
from engine ignition and anything else emitting electrical pulses in the
engine compartment. Since AM is now a 3rd class citizen behind FM and
Satellite channels, there is little emphasis on shielding since the last two
are much less susceptible. If it just started, then it could be a spark
plug going bad, wires going bad or connections to the plugs or spark
boxes/distributor loose. It could also be alternator troubles with a bad
voltage regulator. Make sure your hood is also grounded to the chassis with
a grounding strap.
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