low shower pressure corrects when toilet flushed

[StinkoMan]

Having a bit of a strange problem here that seems to be the opposite of logic...
My shower pressure has lately become very low while the pressure to all other fixtures in the house remains high. Thing is, when the toilet is flushed or a sink turned on the shower pressure increases to its former awesome power, which remains until the toilet valve or other water source shuts off. This makes no sense to me as I'd assume having another fixture running would reduce the volume of water available to the shower and drop the pressure further.
Also, when first turning on the shower, sometimes the pipes below the house make a horrible grinding and groaning sound which again disappears when the toilet is flushed or another sink turned on.
Anyone have any experience or thoughts on this? I need some help as to where to start.

 

[Caduceus]

Check your whole house pressure. Gauges are available that screw onto hose connections. Check the pressure with no other water running in the house and then flush the toilet and see what the pressure gauge reads. Let us know what you find.

 

[Mr_David]

Can you tell us what type of shower valve you have.
If it is a pressure balance valve, the balance spool maybe stuck or malfunctioning.

 

[Caduceus]

I was also thinking along the same line as Mr. David with the pressure balancing spool, but I have never come across a malfunction as described. Then again, Mr. David has seen many more PBS's than I have. I was shooting towards a bad PRV or increase in main pressure without a PRV and the spool is being effected by the change of pressure throughout the house.
Let us know what you have found, this could be educational for others.

 

[Mr_David]

I was just stabbing at it in the dark.
It is an odd symptom.
Probably one that wouldrequire actually being there to look at the problem 1st hand.