The idea of positive reward is one way, and as you are suspecting, it may take some time for extinction to occur.
Rabbits thump when they sense danger, especially from hiss sounds (sounds like a snake! Ah!). So the radiator makes a noise > rabbit thinks it’s dangerous > rabbit reacts.
If you do a positive reward, you’re counter conditioning, meaning the radiator sound will not bring about fear, but it will bring about relaxation because it will be associated with treats.
If you try and let the association extinguish, it means you just expose your rabbit to it enough so that it realizes that no danger actually comes from the radiator, thus taking the “rabbit thinks its dangerous” out of the equation, breaking the link between the radiator sound and the rabbit reacting in this way.
… As you monitor, consider which learning approach is most suitable for your rabbit, because it’s definitely important to have the radiator on so both of you are happy and warm!
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.