Loud Thud experienced today

Today, as I was getting onto an on ramp, and accelerating, the tranny seemed to hesistate, and rev up without going into 2nd/3rd, and then a loud Thud came from the rear. Sacred the living daylights out of me. Any reason for this to happen ? Should i be worried ? No problems in the 60 miles or so I drove after that. no check engine light, but do notice a hisssss sound coming from the engine now, maybe i am just looking for minor probs now.<br><br>Bobmazda...your thoughts ?<br><br>thanks !<br><br>J.A.G

Geee, I don't know. I couldn't even go beyond a guess of a road hazzard??? I can say that the hissss noise if it is when you turn it OFF is the a/c releasing but other than that I don't know....I'd take it in if it does it again...PLUS next service mention it and have 'em check.<br><br>bob