It could be several things. A loose motor mount bolt. Too much valve clearance. Timing chain. A 1987 engine design. I would have them check it out at the service. Probably nothing bad.
I had a 1978 Suzuki DR370 ( bought new ) that had a rattle like that. I tried and tried to find it. Never did. When it finally died, it probably had close to 100,000 miles on it. Well maybe not that many but I rode it every day. The transmission chewed 2nd gear. Kept ridding. Then finally one day the ignition quit. The rattle was still there to the end.