l just noticed this slight noise a few days ago and have since listened intently when driving- it does one knock when l am braking, nearing to a stop. l do not know much about cars, so l am not sure if this is related- but l do need to get a new catalytic converter because one of the flanges has rusted off (it is currently been welded to stay together until l get some money to get it fixed)

So, yeah now l am hearing this, definitely need to get it all looked at.
Also, nearly 2 years ago a mechanic told me l had 50% left on my brakes before l needed new pads?shoes? l dont know, do not remember :)
so any advice?

Brakes have them checked ASAP

OK, two years ago you had 50% left on your brake pads? That is probably a good indication that it is time for new pads. The knocking noise could be a pad that is worn almost completely, of course it would probably squeak too. l think I'd have someone look at the brakes first. That may be all that is needed.