555 design - output to soundcard line-in, and modulating R2

[Roff]

If it only happens when your laptop is on AC power, it sounds like your laptop is not controlling EMI getting back onto the cord. You might benefit from a power cord clip-on ferrite core, similar to this one.

 

[richab2]

Yea... Got a few on some cables for equipment we have around. Might pop one off and try it. If I wanted something packaged with the circuit, would a TPD2F702 be appropriate? The schematic on the datasheet looks pretty straightforward. Do you know if it's possible to assemble this out of components, or is there something that the IC is doing but not showing on the schematic?

 

[Roff]

Yea... Got a few on some cables for equipment we have around. Might pop one off and try it. If I wanted something packaged with the circuit, would a TPD2F702 be appropriate? The schematic on the datasheet looks pretty straightforward. Do you know if it's possible to assemble this out of components, or is there something that the IC is doing but not showing on the schematic?

If the line cord is radiating EMI, you probably can't eliminate it at the output of your circuit with a filter. The EMI can get into the current source and the 555, wreaking havoc with the signal that the 555 generates.
I hate to tell you this, but troubleshooting your circuit without a scope is sorta like swinging at a pinata while blindfolded, especially when you have the pinata and I'm wearing the blindfold.