If your car uses petrol (gasoline) rather than diesel, then the ignition system can potentially generate a lot of noise because of the high-voltage system used to generate the sparks that ignite the fuel.
Electric motors such as those used in the windscreen washer pump and/or wipers (or electric windows, etc.), can also generate noise. This would mean that when you operate the motorised devices, the noise will show on the radio as extra 'hash', and will eventually break squelch if it reaches a high enough level.
All of this noise can get in through the power system (electrical noise), or can be received as electromagnetic radiation across a broad range of frequencies (RF noise). Any electrical noise can be reduced by using choke filters on the power supply to the radio, and RF noise is better eliminated at source.
To answer the question "is it OK for the S/RF light to be illuminated at engine idle?", the answer is that it's not completely unexpected in a noisy environment, but it's something you can probably do something about. The first thing to do (as suggested in a comment to your question) is to disconnect the antenna from the back of the radio, and see if any (even a small amount) of the noise is present when no antenna is connected. If so, then you're in luck, and some choke filters on the power leads where they enter the radio might be able to stop the problem. If not, you have the more difficult task of trying to reduce the noise generated by the multiple systems within your Jeep, at source.