Intermittent slow running bath hot tap?

Every now again the hot water tap on my bath runs very slowly, other times it is fine. At the same time as it runs slowly I usually get a horrendous noise from the pipes in the airing cupboard attached to the hot water tank. Neither of these happens all the time - just every now and again. It appears to only affect or be caused by the bath hot tap - all others seem fine. Any help appreciated