i had a similar problem with my 94, but it wasnt just on the high way it would go away and come back weather i was going 100kmh or just idling.
well anyways mine sounded like it was coming from the passenger side engine compartment (i thought it was my heat shield to).
by chance does your car have ac? and if so does it work?
my ac has never worked and the sound was coming from the ac compressor, so i just got a new belt thats made for cars without ac and put it on.
never herd the noise again
PS. the guy who lives beneath me has a g/f with a 95 cavi and the same thing happened to her car but her ac still worked, so even if your ac works it could be this.