Thumping in 1990 Volvo wagon ? transmission


I have a 1990 Volvo wagon with 351,000 miles, same engine, 2nd transmission. Recently, when driving about 40-60 mph and letting my foot off the gas, there’s a loud drum-tattoo type of noise that sounds as if it’s coming from where the transmission meets the rear axle. What’s going on here? Many thanks. Larry