To get the magnetic field through without interference, to get the full biological effects: AVOID METAL SHEETS
To test the effects on the sound of coils clicking, you can USE A METAL SHEET for a short period of time, that will amplify the clicking effect effect.
But you should not be doing both at the same time.
Smaller bits of metal:, well that depends:
pins and screws: probably minimal effect
larger pins and screws, more of an effect
small belt buckle, probably some effect, but maybe mot much
Texas-sized belt buckle with a tin-man who waves at passers by: probably too much metal, will likely interfere.