Exhaust Issue???

Hi people over the last few days my little GPX 250 has started to pffft pop pffft pfffft wheeeezzzzeee when ever I back off on the throttle or change down and use some compression to slow the bike down or am just going down hill. Now I'm thinking exhaust leak because I haven't lost any power at all and when I'm going up through the gears all is fine. The bike is a 93 GPX 250 any help on what I can check or something that I could try to fix the problem before calling in the big boys would be really great.