Bit of a grey area, because a colleague at work has a Harley that is stupidly noisy, but it obviously gets through an MOT because it's kind of accepted that it's the norm.
Yet as mentioned you get some bikes that are deliberately made to sound obscenely loud which are a complete and utter nuisance to anyone with ears.
I do think though that each rider only hears their own side of things when riding it, whereas what everyone else hears is just appalling, a bit like a tone deaf karaoke singer.
Last night there was a car going up and down the main road next to where i live that was so noisy it was like he never actually had an exhaust on it at all. And worse still he seemed rather proud of it so kept ragging it as well, i've never ever heard such a bad sounding car before, and i've heard some real shitters in my time.