Ok so not really under acceleration but when throttle is pressed 3/4 to all the way down. Does this in neutral, any gear, at any speed, engine hot or cold. Engine is 4.9l transmission is m5r2 Mazda 5 speed. Its just a fast paced rattling sound, almost like a rattle snake or something. Any one else had this problem?

 

Does it do it in neutral while stopped? If so, have someone put the parking brake on and get under the truck and have them rev it up and see if you can locate the rattle. Exhaust is usually the culprit. Since your temps are in the low triple digits it should be fun. We are melting here. Sandy