Threatening Black Van
 

About fifty years ago I saw, in a UK city independent cinema, a film in which a small group of people travelled by road for three or four days and nights, stopping only at service stations and motels. All the time they were tracked at a distance by a large black van. it had no windows, or they were blacked out, and every time we saw it there was a deep continuous note on the sound track. We didn't see the driver. I can't remember the story, if there was one, but it became spooky with repetition. Eventually there turned out to be a simple reason for it all. The 'star' was the van with its threatening sound.
Not much to go on but perhaps someone can remember it?
Thanks.

Re: Threatening Black Van
 

Sounds like Slither (1973) crossed with The Car (1977).

Re: Threatening Black Van
 

It could be DEATH CAR ON THE FREEWAY (1979) directed by Hal Needham.