If it worked fine before you added the servos, then I would first suspect the servos. I have several HFS-04MG receivers, and have had ZERO trouble with them.

Pull the servos off, one at a time, until the chattering stops. The servo that stops the chattering is the bad servo. Replace it.

Unfortunately, the equipment necessary to scientifically trace these kinds of problems is prohibitively expensive, and requires a college degree to operate... You have to go to the old fashioned trial-and-error method.