Why use multiply if you want noise added? Why not just use add noise and then offset down to bring the surface back down? For example, using Filter>>Noise>>Fractal Noise with an Operation of Add and an Amplitude of 100 will add numbers from 0 to 100 to your surface followed by using Filter>>Fill>>Set Value with an Operation of Subtract and a value of 50. These two operation have a net effect of adding fractal noise ranging from -50 to +50 onto your existing surface.