I am a service engineer for a company that manufactures lab equipment and I wanted to ask for electrical engineering help with a problem. We make a product that consists of two parts, one of which is a heatable water bath. I have a customer who since he has had the unit, has had a problem where the instrument emits a loud noise only when the heating elements are turned on. I took a microphone to the instrument hooked up to my scope on FFT, and measured the noise to be at 3.1 KHz. The heating control PCB and power supply PCB are both located in the heating bath part of the unit. In order to resolve this, we replaced the heating bath with a new one, assuming the noise was coming from one of the PCBs. The customer plugged the new one heating bath in and it is still making the noise when the heating elements are on. I took the old heating bath home, and when I plugged it in I do not hear the noise. The heating bath has resistive elements which are controlled by a triac off of 110 V . The only variable that makes sense is that the noise is caused by something with the power supply in his lab? If anyone has any insight it would be much appreciated. What could cause a loud 3.1 KHz tone to be produced when a heating element is powered on in one environment, but not in another? The rest of the instrument does not contain any electrical components related to the heating circuit.