It’s no secret that if your guitar gets close to a loud amp, you’ll get feedback — the sound from the amp causes the guitar’s strings to vibrate, creating a feedback loop. But if you want to get feedback at lower volume, it can be hit and miss. One thing you can try is to put a high-gain distortion pedal between your guitar’s output and your amp’s input. The idea is to add enough saturation and compression to the guitar’s signal to cause it to sustain for an extended period so that the string will have time to vibrate in response to the sound coming from the amplifier. It will also help to get the guitar close to the amp’s speaker. Another trick is to put the headstock of the guitar against the side of the speaker cabinet.
It’s difficult to get controllable feedback at “bedroom” levels, but with sufficient gain and close proximity to (or even contact with) your amp, you should be able to get singing, controllable feedback at low volume. Just turn the pedal on when you want feedback, and turn it off when you’re done.