i have a 87 b2600 carbed automatic. everything was working fine. i could actually drive it. i put a mechanical oil pressure and temp gauge in it to keep an eye on things. the other day i was driving home with no problems and then all the sudden when i let off the gas there is a very loud noise that comes from the motor. its kinda like a knock. but loud and fast. the truck still ran fine had oil pressure and was running cool. i drove it back to the house and let it set. cam out next day and started right up. little knock on start up but ran perfect. if i rev the engine above 2500 and then let off it knocks real bad again but it drives like a dream. i am stumped anything will help.

 

It sounds like it could be a piston hitting the head. I had an Eclipse with a 2.0 that did the same thing. Anyway I know it's not the best suggestion but that is what mine sounded like when it happend.

 

It sure sounds like you have a rod bearing problem which has that classic sound. It gets louder when you let off the gas. The Mazda engines are not the interference type and the pistons will not strike the valves. I know what the previous poster was saying as I used to work on those Mitsu engines.