I just noticed this slight noise a few days ago and have since listened intently when driving- it does one knock when i am braking, nearing to a stop. I don't know much about cars, so I am not sure if this is related- but I do need to get a new catalytic converter because one of the flanges has rusted off (it's currently been welded to stay together until I get some money to get it fixed)
So, yeah now I am hearing this, definitely need to get it all looked at.
Also, nearly 2 years ago a mechanic told me I had 50% left on my brakes before I needed new pads?shoes? i dont know, don't remember

so any advice?