Question: I get this really bad feedback sounding noise when I play stuff like this, from the beginning of the Free Bird solo, 
shown in tab:

```
1  ----18---18---18---------
2  -18---18---18---------

```

(All notes on second string played with a full bend.)

It's there on any amount of distortion, but not when I play clean. Is it a problem with the amp? If so, what can I do about it?

Answer: 
This could be a number of problems. 

1. Dissonance: This, in basic terms, is when two notes do not sound good together, and clash producing an unpleasant noise. However I have tried this out on my guitar and amp and I do not get any dissonance, so it may not be that.
2. Amp/Speaker Problems: I have found, particularly with Line 6 Amps and Speakers, that when the bass and mid knobs are turned very high, the speaker cannot cope very well and produces and grinding feedbacky boom.
3. Tuning problems: It may sound stupid and patronising of me, but are you perfectly in tune? According to both ear and tuner? If so, your guitar's intonation may be out. Any slight fault in an intonation setup causes the guitar to be out of tune with itself, and therefore notes that usually harmonise well may suddenly sound dissonant. This is greatly accentuated at higher frets, where strings will sound even worse if the intonation is out.
[See this previous question](https://music.stackexchange.com/questions/1604/what-is-intonation-and-how-can-i-find-out-if-my-guitar-has-correct-intonation) for help about intonation.

Distortion greatly increases the unpleasant dissonant sound, so that may explain why you do not find you have the problem on a clean setting. 

Hope this helps.
Is the amp solid state, digital, or tube? Is the volume high or low, does reduce in gain and/or decrease with volume? Can you reproduce the sound without bending(could be an artifact bends not in tune). Is your guitar intonated properly?

Note that the guitar uses equal temperament and not all notes are in tube. m3rd's with a lot of distortion of a specific type can sound pretty bad. With class B tube amps you must turn them up because they have intermodulation distortion which is almost at a constant volume but can be masked with volume. That is, the louder the amp the less you will hear this type of distortion which is very annoying. Solid State can suffer from this too but digital won't.

Distortion adds harshness to a sound and it can be emphasized with certain notes and intervals. Distortion can come from a wide variety of places in an amp. Some good and some bad. Sometimes the "bad" distortion can be good in some context.

Naming the amp you have, if you can reproduce this sound on the clean channel with slight over drive, and a clip of the sound will help(although I think I know exactly what you are getting at as I have a similar issue with my amp ATM and I'm in the process of tweaking it to get a smoother distortion).