I had a similar sound on my '93, a noise that sounded like it was coming from behind the engine, but it was only when the engine was under a hard load, like high throttle in a low gear or going up a hill. I cleaned out my intake with carb cleaner and tightened all the hoses and stuff associated with the intake and it went away. I think it was something dirty in the intake. But it's hard to tell what yours might be without being there.