gain of 100, and going into what is already a mic preamp in the computer........ going with a much higher value input resistor, or going with a noninverting amp would be a good start. knock the gain down to more like 10. also if you stay with the inverting amp, bypass those 47k resistors with a couple of 1uf or higher caps. you could be amplifying thermal noise from the 47k resistors, which will definitely add to the noise. you are probably also amplifying noise from the 7809.... is the 7809 properly bypassed with 10uf caps? i've seen 78xx regulators get noisy when the bypass caps dry out, because the regulator is primarily an "amplified zener", and the zener used as a voltage reference inside them creates a fair amount of noise, which is one of the reasons for adding the bypass caps at the input and output of the regulator. so you actually have several sources of noise here. the regulator and the voltage divider for the noninverting input you can do something about with a few electrolytic caps, but you can't do much about the input transistors of the op amp. an LF351 is (compared to a 741) a low noise device, but there are other devices out there that put the LF351 to shame. cutting the gain down to 10 rather than 100 will reduce the noise quite a bit. from what i see you're using an electret with a built-in jfet preamp (which is the only reason you would feed it with DC), so you really don't need a gain of 100 anyway. your PC input also has a mic preamp, so a gain of 100 stage before a mic preamp is going to be very noisy. go with a gain of 10 (replace the 1k resistor with a 10k), and if you want it to be variable gain (not neccesary, just adds a bit of flexibility to the circuit), replace the 100k resistor with a 100k pot. that way if you find a gain of 10 too much, you can turn it down from there.