I second coyt about checking the legality. From what you say, I take it that this is a 2-family house, with just you upstairs and the landlord downstairs. Did your parents sign a lease? If so, they should check to see what it says about pets. Did your landlord know that you'd gotten cats in August?
Of course, you don't want this to get contentious, but if he knows that you're looking for a house, maybe you could just ask him nicely to be willing to put up with the kitties until you're able to find a house.
At the very least, I think you should ask what kinds of noise they're making. Sometimes things are noisier than you'd think, but he should at least give you an explanation and a chance to correct the problem before telling you that you can't have the cats.
Is it thumps? Is it meowing? Ball toys? If you've got wood floors and they're jumping down from things and making thumping noises, maybe you can get a couple of rugs to muffle the thumps. Or if they're chasing toys that have bells, just take them away at night, or get toys that don't make noise.
The landlord just seems to be a cat-unfriendly person who's making excuses.