We just purchased a home built in the 50's, and the grill for the return air vent in the hallway is sealed with tape and aluminum foil. It seems to be pretty tight, as if whoever sealed it had a really good reason to do so. What would the reason be for doing this? We can't decide if we are going to take it off for fear of some unknown problem. Thanks!

There could be many reasons that a person might do this. So I will name a couple that come to mind. It could be that the underground duct had collapsed and since no air was going through it might have started to have an odor. Second it could be that they were very light sleepers and the noise form the vent kept them awake.
I would take the tape and foil off and see what developes. You can always put it back if needed.

I would look at your air circulation. Could be that too much air was being drawn in there and not enough elsewhere. If that is the case, a new grill with a damper you can close might be more presentable. If you are really sure you will never need it, you could dry wall over it.