SM57, six inches from and slightly below the bridge, pointed up at the soundhole of my friend's very nice sounding Larrivee. I noticed that taking a little out of the 1-2kHz range improved the track quite a bit. When I swept through it with the parametric EQ at +18dB, this really horrible, almost feedbacky resonance jumped out, centered and most prominent at 1.5kHz and extending down to 1.1 and up to 2.2. It's not on the other guitar track I recorded that day with him (same guitar), it's not on the vocal track recorded at the same time (which has guitar bleed) and it's not on any of the other acoustic guitar tracks littering my hard drive (the ones I haven't thrown away). Obviously I'll be watching this frequency range like a hawk in the future, but it'd be nice to figure out possible sources for this noise. I've come up with a few: 1) It's in the guitar. The track with the noise was in a different key from the one without the noise. In that key, the guitar resonates in a way that is not noticeable in the room but that interacts with the recording chain in an unpleasant way. I can check for this the next time he comes up to record. 2) It's in the chain. Electrical noise from the computer (it's the only thing plugged into the wall; my Duet is powered from it), or interference from somewhere, or a crappy Garageband parametric EQ. But why would it just be on this one track, and not on the vocal too? 3) It's in the room. Something (water heater, fridge, monitor) kicked on, and only the 57 was in position to pick it up. If this was the case, though, I'd expect to find the noise on more than one track, since this stuff kicks on and off fairly regularly. 4) Aliens flew over my house, and what I am hearing is the tortured screams of their human lab rats. The guitar mic was pointed up, and the vocal mic was pointed down. This explains why the squeal isn't on the vocal track. Any ideas?