I remember little about the film, but it had a lasting impression. It was B+W. I saw it on TV in the early '60s, therefore I think it was a 1950's movie.
The scene I recall vividly: two guys enter the factory to investigate suspicious behavior regarding missing persons. Down in the basement, under large overhead pipes, they can hear objects shoot through the pipes. One of the pipes is leaking. One guy lets some drip onto his hand, only to find the fluid is human blood.
They realize that the whooshing noises coming from the pipes are of human bodies being moved through the pipes from one factory area to another. They decide that the people that were missing are ending up here to be processed. I can't recall what for, or by whom, but probably as food (for aliens?).
They run off in vintage looking vehicles. I seem to recall that they were British, but I can't be sure.
That one scene has remained with me. Every time I see facilities with pipes, I refer to is as an alien human processing plant.
Can anyone help me identify the movie?