Amplifier HISSIN Noise ? What Causes this ??

Im New Here, Im a Novice DIY Electronic hobbyist ...I have a small shop with Meters, Scopes, Tools ......etc

I Have a Behringer Loudspeaker, a B615D 1500 Watt PA Speaker with Amp Built in. When you power up the Speaker ( with no input ) the Louder the Volume Knob goes, The Louder "white noise hissing " it does. The customer says never did this before.

I Checked for the Caps being buldged, or any cold solder joints. What Causes or most likely creates this Noise ?? is there testing I Can do ??
Resistors etc can cause noise.

Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltage. Thermal noise is present in all electrical circuits, and in sensitive electronic equipment such as radio receivers can drown out weak signals, and can be the limiting factor on sensitivity of an electrical measuring instrument. Thermal noise increases with temperature. Some sensitive electronic equipment such as radio telescope receivers are cooled to cryogenic temperatures to reduce thermal noise in their circuits. The generic, statistical physical derivation of this noise is called the fluctuation-dissipation theorem, where generalized impedance or generalized susceptibility is used to characterize the medium.

Thermal noise in an ideal resistor is approximately white, meaning that the power spectral density is nearly constant throughout the frequency spectrum (however see the section below on extremely high frequencies). When limited to a finite bandwidth, thermal noise has a nearly Gaussian amplitude distribution.[1]
Without any input plugged in the the amplifier sees all the noise from its input load resistance which is usually 100k or so - this will normally be quite audible.

With a signal source plugged in and powered up (but not playing anything) the hiss should be much lesss.

Note that some modern mp3 players and phones etc power down their audio output stages when not in use, leading to very obvious hiss. No pro audio device should display this woeful behaviour though.

A work-around is a low impedance load resistor on the input (say 1k or so) to limit the effect.
Bear in mind, on the Behringers when the knob is above middle position You are at mic level amplification. It's perfectly normal. Behringer stuff is well known for hissing, but that isn't hiss. It's noise. It shouldn't make noise under middle position.
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