I Love heavy rain. I was coming back from a bike ride yesterday and it started bucketing, I couldn't stop smiling! Just the noise, the feel... It's great to be in it for short periods, and then watch it from behind a window afterwards (if it's too cold then not so much, but at he moment when it's reasonably warm it's great).
As I rode back through the estate the rain was generating a mist of spray near the ground that just made the whole place look like it should be inhabited by fairies - it was lovely.
So I suppose when I'm outside it's not so much relaxing as interesting and exhilarating (miserable if it's cold though), but inside yes it can be very relaxing if I allow myself to just watch and listen. Soothingly repetitive.