when i play my brass feadog D.. after i played i can sorta taste the metal… am i supoose to clean the whistle after i played it.. would it go rusty??
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Which end are you blowing into?
I never had that problem. I clean mine whenever I feel in the mood.
I haven’t experienced that with mine, either before or after changing to a Whitecap mouthpiece.
Brass doesn’t exactly “rust”, but if your lips turn green, that’ll be a clue that some undesirable chemical process is going on. Nice for St. Patrick’s Day, though. 
I made a little swab for mine by using a hacksaw to cut an inch-long slot in the end of a 1/4" dowel (14 inches long, with the ends sanded round). I wedge a little strip of cotton T-shirt material in the slot and swirl it through the barrel. I use that before putting any whistle away. (I also have a piece of guitar low E string with a hook at the end, for when the cloth comes out of the slot in the upper reaches of the barrel.)
For the mouthpiece, I use a narrow strip of cardboard with a drop or two of liquid dish soap on it. I only use that when I start having trouble with clogging.
Sequel: which end do you suck from?
Seriously
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do you tend to “steal breathes” by breathing through the whistle?
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If not, it may be an acidic perspiration from your fingers. Wash the whistle, spray the tube it with a household wax (Johnson Pliz rings a bell or is it strictly Euro market?) let dry a bit than polish: this will isolate the metal from sweat.
If this happened to me
I would consult my medical practitioner
or naturopath to test my copper and zinc levels .
It used to happen to me. I think it’s a matter of excessive saliva and breathing in through the whistle. Cleaning the whistle helped too.