Can Computers add noise to home electrical system

The only question I would ask is are the fan motors AC or DC. How old is all the house wiring. Is the house ground intact ok that is more than one. I run a dedicated line with a power conditioner and have no problems.

Yup, I got 2 20amp lines and I couldn't be more pleased by the improvement. $500 bucks and it's like you upgraded everything!

I suspect computers DO add noise, despite dedicated lines. I can certainly vouch for microwave ovens adding a discernible grain to the high frequencies, dedicated lines or not! I could always tell when the microwave was still plugged in because the harmonics were stilted -- if they were there at all! And this was on two different coasts, so it sure isn't a local effect.
A dedicated line, contrary to stated opinion, does NOT eliminate interference from household lines on non-dedicated lines. Try turning off your cable box and listening to the same cut again. Or, as I suggested, the microwave. Or, your 2MHz cordless phone. It's distressing how many appliances that were not around in the 80s or even early 90s can now inject RFI or EMI into a system. If anyone knows a cure, let us all know!

Interestingly, when I had a system set up in the basement, and had a separate circuit box for the dedicated lines -- and the lines did not run through the main floor of the house, it seemed quieter. Operative word: SEEMED. There was no way to compare, although I did unplug the freezer in the basement, but that was to eliminate the freezer motor hum. I suppose I could have tested more, but that wasn't the point at the time.
Do the microwave test and see for yourself.