Shower - hot to cold

Styx555

HI, over the last few months have had a water issue in my standing shower. When turning on the water to hot, sometimes it works, other times only cold water. If I turn the washroom sink to hot, it always works and seems to also kick start the shower. All other taps in the house have no isues.

- the shower is a single valve Moen
- I have a tankless hot water heater
- no other taps/faucets seem to be affected

Last night I changed the Moen valve, but this didn't fix the problem.

I also tried installing a new shower head, however when I turned on the water, heard a fog-horn type of sound and rattling of the pipes. Water pressure was also low (I currently have a water saver shower head).
With no shower head, prerssure seems fine.

Thanks