Follow the noise,- find the blinker unit and pull it out! Then turn on the 4ways, and follow the noise to this blinker box, pull it out and put in where you just pulled the "blinker one" out of, --put "old blinker" unit in 4way plug! If it is an "idiosyncroncy" of the "blinker box" itself, it may work just fine in 4way, -- and you don't use it much anyway, -- And if the turn siganl works fine, - no more problems(?) -- then you have "solved the problem" and not even spent any money!
I also may be possible to stop this problem by putting a capacitor across the two terminals (or from "flashing side" to ground)! Note one side is "hot whenever the turn signal is on, the other is interrrupted when bi-metal unit "opens"). Go to radio shack, -or steal one from an old dead radio or tv set, -- get regular capacitor (not eletrolytic), - anything around .5 mfd and (rated) voltage above above 20 volts will do! Note that you can put several in parallel to increase "capacitance rating" -- (IE:-(2X) - .25 mf in prallel equals .5 mf too)! Or any combination, - does not have to be exact total , just close one way or the other! These are not polarized (electrolytics are), -- so it doesn't matter which side you hook to which! ..... This removes "spike" when contacts "make" and will stabalize circuit somewhat, -- (especially if it makes noise in radio too!) This commonly referred to as a "filter",-- and removes static from electrical units to make less "static" in radio (especially AM!
If box makes tha noise in 4way( and blinker sounds ok now)--get a new box for there,-- if the blinker still makes this noise,-- with other unit in it,--Try to buy a different type of "box" than the one you are having trouble with, if "electronic, - try an "old fashioned mechanical one", -- if mechanical one try an electronc one this time- (the electronic ones cost about 4X the price of the old style one!) old style is just a "bi-metal unit ' with a heatable element in it! When it gets hot, the contacts open and the blinker light no longer has power available - when it cools off, the contacts close and light comes on again, - (simple and to the point)! -- Electronic one has a transitorized timer circuit, and the rate blinking is more controlled, -- but because of timer circuit, they are much more expensive!
Note, - if turn signal blinker ever fails you and you need one quick, the 4 way uses the same"box", - and you can switch and go on your way, -- you hardly ever use the 4way blinkers anyway, so immediate relacement of box is not a "big deal"!