1999 Subaru Impreza question

I feel rumbling rattling vibrating in the engine when I use the gas pedal especially when I accelerate. Sounds like a train engine. Comes and goes. When the gas pedal is released during the rattling, it stops vibrating but happens again when I press the gas pedal. The car still runs despite this but it does sounds very annoying and feels abnormal. Any answers?

Thanks.
Roy