Dramatically lower flow rate or an error with temps measurement. Clogged waterblocks?
Mkay, you are describing your friends' problem, so let me tell my friend's story
One of my penmates had his flow totally stuck after only adding new Watercool res, a new additional 360 rad and ZMT tubes. He tried all, thinking he somehow made a mistake somewhere in flow direction, or there is large air bubbles in both his pumps. Bled the loop multiple times, tried it both with external Mora and without.
I said him that he checked everything except opening hs waterblocks. He resisted for some time, trying some other things (luckily he had a mps400 that showed total zeroes, and in his reservoirs he saw how really thin was incoming stream)
Finally, he opened his blocks and both were totally clogged. He cleaned them, restart his loop and flow was perfect, better then before.
You should also be thinking that is a bit strange behavior if he only changed reservoir and temps (probably coolant temp) rise 6c having all configuration the same.
Regarding noise, an award winning overclocker and mechanic should clearly understand that reservoir itself cannot be a noise source. So, more noisy operation is due to new reservoir mount, rather bad pump vibration dampening. It is also possible (but not likely) that pump was installed incorrectly in a new Ultitube. Usually you need to use provided with res o-ring for the pump, to get correct gaps inside its working chamber. Also it worth mentioning, that if there is something clogged in his loop, it is normal that pump is running louder then if the loop is perfectly clean and flow rate is high (meaning loop having low restriction would be more silent, then restrictive loop with the same pump and its operating power)
If there is some way to check how's current flow rate, I'd checked it first. And, just like my friend, checked my blocks.