I originally posted this in the tech discussion forum, but it took a while for me to get a recording of the sound. I thought this might be a better forum for it. My '05 has developed a strange noise from the engine compartment. It sounds like a pump or valve cycling on and off, about once per second. It comes up seemingly randomly and might last for a few seconds or near 30 seconds, engine on or off. It has been happening for the last 30k miles or so. I'm fairly certain that it hasn't always made the noise, as it is pretty noticeable in the cab. I also noticed that my in-law's '04 has the same noise, but less frequently. It's definitely coming from the thermos bottle area. I can feel it when I touch the plastic tee on the bottom of the thermos bottle. I connected my Autoenginuity scan tool, and cycled the water pump and water control valve(s). The water pump makes a nice even whirring sound, and the noise comes up after cycling the water valve. No MIL for a failed coolant flow control valve. Any thoughts? From the cab, the clicking in the recording isn't really evident, but it is when you get up close to it. Does this noise sound familiar to anyone?