Hoping some experts can chime in and help out. Both my plumber and I are stumped. Here's what we did: 1. Replaced two 50 gallon water heaters in the attic with a 75 gallon in the garage (18" off the floor). The 75 gallon was directly below one of the old 50's in the attic, and we ran connected the hot water line from the other old 50 through the attic approx. 50 feet (wrapped in insulation) to the hot water line exiting the new 75 gallon. Our house is single story, has no pressure reducing valve and was built in the 90's. 2. Installed a recirculating pump and two transfer valves at the farthest points in the house. 3. Immediately after this work was complete, we experienced water hammer at both the new 75 gallon AND near one of the transfer valves (transfer valve installed at a bathroom sink but the hammer was in the nearby shower wall) when running the washing machine. We did not have water hammer with anything other than the washing machine. 4. The washing machine is close to the 75 gallon but far away (approx. 50 ft.) from the transfer valve where the hammer occurs. The washing machine already has hammer arrestors on both the hot and cold sides. We did not change these. 4. We installed an expansion tank on the cold water line at the new 75 gallon. Our house water pressure is ~60 psi, and we originally set the tank pressure at 60 psi which worsened the hammer and even caused sinks and toilets to create hammer. We lowered the tank pressure to 55 psi per the tank manufacturer, and the water hammer is back to occurring only when the washing machine runs. 5. We replaced the recirculating pump which did not affect the hammer. 6. We replaced the transfer valves which did not affect the hammer. 7. We drained and then flushed the water lines in the house. The hammer disappeared when running the washer but just for one day. We did wash the next day, and the hammer returned. The expansion tank appears to have no effect on the hammer, and we never experienced any water hammer prior to combining the two water heaters and relocating down from the attic into the garage. Can anyone offer insight as to why this is causing the hammer and how to get rid of it? We've spent way too long on this issue and don't know where to go from here. Thanks in advance for your help!