Let me start off by saying I know nothing about audio. I'm getting these distortions on my new 5.1 system under certain circumstances. They're rare but I still want to get rid of them because when they come up it's distracting. The distortions sound 'electrical' when they happen. I remember once when I was getting distortions on another system I lowered the treble and that got rid of it. Does treble cause all distortions? Can bass do it?

Please give more information about under which circumstances you get these distortions.
Do you listen quite loud with bass level tuned up?
Do you hear the distortion from the subwoofer or from the other speakers?
Are they gone when you turn your equipment more silent?

... and first of all, welcome to HydrogenAudio

Like I said I don't know much about audio so I wouldn't know how to explain under which audio circumstances it happens. But it is rare.

Can someone just tell me if it's possible for bass to cause distortion? From my experience it's only the treble setting that has to be lowered to lessen distortion. Although judging from your post it seems it is possible for bass to cause distortion too. Next time I come across this I'll be sure to just fiddle with all the settings.

neight,

you don't have to fiddle with all settings.

As you already stated, the distortions get better when you lower treble setting.
Generally distortions are caused either by a clipping amplifier or loudspeakers mechanically strained too much. In your case I think it is the amplifier, that's why I suggested to just turn it a bit less loud.
In Music and Movies, most energy is needed for low frequencies. Our hearing is quite insensitive for low frequencies, so a lot of power is needed to make them equally loud as the rest.
So if you turn up bass the amplifier will quickly clip, even if it is not too loud. When you make frequency response linear, clipping will no longer occur.
For more information about clipping, read the Wikipedia article

There are all kinds of distortion and it can come from many different sources... If the distortion is present in the source (on the CD, DVD, broadcast, MP3, etc.) there is not much you can do about it.

The audio travels through a "chain" of equipment and processes before it hits your ears. Distortion (and noise) can come from any of the "links" in the chain.

Sometimes the only way to find the source of the distortion is to swap components... If you've got distortion coming from the left speaker, you can swap left & right speakers... In other cases, you might have to try a different set of speakers, a different amplifier, etc.

Clipping (the most common type of distortion) does generate harmonics (high frequencies). So, if clipping is present in the source, turning down the treble will reduce the audible distortion.

But if your power amplifer is clipping, the distortion is being introduced after the treble control, and in that case, the treble control is not likely to have much affect. In some cases, the high-power harmonics generated by clipping can burn out a tweeter... even if it's the bass that's distorting!

Speaker rattling is probably the 2nd most common type of distortion.

If it's only distorting when you 'turn it up' it could be your power amplifier clipping, or it could be your speaker rattling.

Low-bitrate MP3s can have a unique type of distortion that's hard to describe (they can sound something like a cell phone).

Very good post DVDdoug just what I wanted. Thanks.

What does it mean when a speaker has rattling? I think I narrowed down the distortion to one of the rear speakers.

It could mean a loose part, such as a screw, or it might mean a tear in the speaker cone, but I think most likely the voice coil is rubbing against the surrounding magnet.