most likely you're hearing the 4558 op amps. as i asked how loud is the noise? input preamps can create some noise and nothing actually be wrong with them. the real way to know for sure is to measure the noise voltage at the speaker and compare it to what the full power voltage would be at the speaker, which would give you a ratio. the ratio can be converted to whatever flavor of dB they used in the spec sheet for the amp and compared against the specs for the amp. being a small practice amp, it's probably not even in the spec sheet... RC4558 op amps are spec'ed for 10nV/√Hz. so for a gain of 100 input stage, at 1kHz, the noise voltage would be 31mV, and in a 10W amp, the voltage gain is probably about 10, so that would be 310mV at the speaker. that definitely would be audible. if the op amps are old or there are large value carbon composition resistors used in the input circuit, you could have a lot more noise than the basic noise floor of the op amp. also be aware that semiconductor junctions begin to develop more noise before they fail, so you may have an op amp in the process of failing (NASA uses this fact in their reliability program).