Many hams find that their HF mobiles receive more noise than they do usable signals. Common noise sources include the ignition system and alternator.
Spark plug noise is propagated through ungrounded body panels and the exhaust system, which is normally suspended from rubber vibration dampers. The rubberized supports insulate the exhaust from ground, essentially creating a radiating spark antenna.
To deal with spark noise, use tinned copper braid to electrically connect the hood (or bonnet) to the firewall of the vehicle. This will turn your hood into a Faraday shield. Stainless steel clamps of the proper size may be used to clamp braid to the exhaust system, (typically in two places- one near the front and one in the rear) thereby reducing the radiation of noise. DX Engineering DXE-TCB-050 Tinned Copper Braid is a half-inch wide and ideal for this purpose.
For alternator noise, products like the Kenwood PG-3B Inline DC Noise Filter will eliminate noise from DC supply lines in radios up to 100 watts output. As a fringe benefit, noises other than alternator noise, like that of onboard system control computers, will also be reduced.
You really don’t have to suffer with vehicle-generated HF noise. A little elbow grease and some handy tools and parts will go a long way toward making HF mobile operation more enjoyable.