I have an Amer. Std. Heritage 15 heat pump that has started to develop quite an annoying noise. The outdoor metal front louvered condensor coil guard is vibrating slightly back and forth when the fan is on. It is definitely because of the air flow not the compressor. The left,right and rear covers do not vibrate at all. If I press on the cover in the middle of course the noise will stop. Has anyone ever seen this before? If so is there a fix? Service bulletin?