I made a short video of the issue relieving noise that is made about every 25 minutes
https://www.youtube.com/watch?v=42TkKYVIsxw&feature=youtube_gdata_player

Hi Pjt123

Although the sound appears to be at the expansion tank or water heater, the only thing I know of that cycles (and can sound) like that is a leaky toilet. Here, if the toilet tank flapper is defective it can let water slowly leak by which results in the toilet periodically refilling/cycling and that could be the sound we are hearing. To test this, take some dark food coloring and drip 20-25 drops into your toilet TANK and then check the toilet BOWl every 20-25 minutes (especially when you hear the sound) to see if the color is showing up. If colored water shows up in the toilet bowl then the toilet tank FLAPPER will need to be replaced, and hopefully, that will solve the noise issue!

If no coloring shows up in the toilet bowl, pop back and we'll see what we can think of next, OK?

Mark