If I turn up the volume on my Frontman 10g more than 4-5,the amp starts buzzing every time I play the thick strings. Thank you!

Get inside it and check the connections to the plug especially any shielding / braiding. Resolder the connections.

To lower the action on an acoustic guitar:

Usually just the saddle needs reducing.

1. Remove strings (or you may be able to just loosen them). Remove saddle, sand down the bottom a little at a time. Restring and check action.

The action can be set as low as poss without any fret buzzing or note choking on all strings and on all frets.

2. Repeat 1

If the action remains too high then there are other techniques.

Later ...

Find another speaker to check it with. I've had several fender amps to blow their speakers during loud playing.

... do you, by any chance, have non humbucking pick ups on your guitar? Just curious...

Simple: connect the headphones out (3.5mm jack) to the input jack of your fender (6.3 jack). By doing this you will loose one channel so you better generate a mono signal inside your computer before. Electrically this is no problem, nothing will be damaged. Turn the gain and volume knobs down and apply a signal from your Mac. Gently increase gain and volume. Et voila wonderful mono sound through a guitar amp

There is something resonating with that note frquency. Often the grille or wiring inside will rattle at certain frequencies. You need a sustained source of these frequencies to find what is causing the noise.

The schematic will be VERY close to this:

http://elektrotanya.com/fender_frontman-15b_sch.pdf/download.html

Scroll down to "Get Manual" on the web page and click on it to download.

The 10G may use a lesser power output chip but rest should be the same.

If the ringing only occurs with a specific range of notes it may be something rattling in the amp loose or speaker enclosure.
It could also be feedback occurring at certain frequencies - does the ringing stop if you turn the volume down a bit or stand in a different position in relation to the amp?
Could be a problem with your guitar? Maybe the pick up height needs adjusting in relation to the E string or is it the string buzzing against a fret - indicating a need for adjusting the bridge height or trussrod settings?

If you don't want distortion, get rid of the overdrive pedal... Twangy USUALLY comes from the GUITAR and NOT the amp... test it on a different amp to decide... Buzzing CAN be caused by "fret buzz" which is due to IMPROPER guitar setup such as neck bend and string height.
Start using ONLY clean channel on the amp and NO reverb. If you verify that the guitar sounds bad on a different amp, take it in to have it setup properly.

When i turn on my 212R it turns on in drive channel. if footswith is plugged in, then it turns on in the clean channel, but after switching in drive it is impossible to switch it back in clean channel. What could be wrong? Swith position doesnt change anything and it seems like switch is OK.