Hi this is the second time this happens to my heli I just started working on a kalt AH-1G Cobra and I installed an OS 61 rear exhaust engine and due to lack of space I'm using an after market exhaust like the tatoone or pitts muffler, the sound is too loud and I noticed that the throtle servo shakes like its receiving interference when the engine is running and works fine no movement at all when the engine stopped, I checked the servo arm, the servo screws it seems like the sound of the muffler is causing this problem. I checked the battery and the receiver, both were just used in an airplane, the radio system is the Futaba super 8 with receiver 148fp is there any suggestions?
thank you?

Back in my plane days I used several of the pits style mufflers, and none of them caused ant radio interfearance. Check again for any loose bolts or clamps that could be rubbing. Other than that make sure the engine is running smooth. Is it only affecting the throttle? If so maybe it is just the servo, or the throttle channel, you can check simply by swapping in another servo.

The servo may have a dirty or worn pot and the vibration is affecting it. How old is the servo?

Thank you for the suggestions I'll try to change the servo and check for loose conections this servo is not too old and it was working fine in a plane, like I said only happens when the engine is running so may be the conection is bad, I hate to take it a part since it took me almost 8 hours to fit the mechanics in the body but I rather do that to see it die before the engine is broken in. Well thank you I'll post the results soon.