As mentioned above... changes in environment, from shipping to a new home or seasonal fluctuations in temp or RH, can affect your M-20. In fact, I have to tweak the trusses on several of my guitars twice/year... in late fall as my cave cools down to its winter time temp of 60F, and then in late spring back to its summertime 75F.
Here's what I would do...
(1) Check the neck relief on your M-20. Put a capo on the 1st fret. Then fret the 6th (low E) string at the 14th fret. Now, look at the 7th fret... there should be a tiny (half the dia of a high E string) gap between the fret and the string. Rather than looking, I usually tap the string at the 7th fret, and can tell by the string movement and clicking sound how much gap there is. Proceed to check all the strings... they should all have a little gap. If not, loosen the truss nut (CCW) 1/4 turn, wait, and test again after a couple hours. Loosening the truss will allow the string tension to bow the neck forward a little... creating more relief. This should always be done carefully, and just a little (1/4th turn) at a time. A small adjustment of the truss can have a dramatic effect. Repeat this as necessary uuntil you have a small gap... The converse... if you have a large gap (which is not a cause of buzzing, but of sore fingers!), tighten the truss CW 1/4 turn...
(2) Once the relief is set... remove the capo and check the "action" (string height) at the 12th fret. Different peple have different opinions on how high the "action" at the 12th fret should be. On my dreads, the low E is just a shade over 3/32 above the 12th fret. The high E is on my dreads is usually between 2/32nds and 5/64ths above the 12th fret. A heavy strummer might like the action a little higher... a finesse fingerstylist might like it a smidge lower. Mine is a compromise... easy to fret, but if I play too hard I will buzz a bit. ACtion is usually adjusted by adjusting the saddle height (or in some cases, the nut)... something that I have (so far) left up to my tech/luthier... but I like to have a good idea what I want before I take my guitars to him.
(3) If the relief and action seem fine, but you still have buzzing, I'd check for a high fret. Sometimes I can narrow it down to a particular string and or fret by just playing each string at every fret up the neck, listening for the appearance and disappearance of the buzz. Sometmes, just sighting down the neck can reveal a hump in the frets. But another easy test uses a credit card (but doesn't cost anything!)...
Take a nice straightedge (the edge of a credit card works great) and span the first three frets (use the long edge of the card). Try and rock the card back and forth like a teeter-totter (see-saw). It SHOUDLN'T rock. If it rocks, the fret in the middle is high. Then, do the same thing spanning frets 2-4... then 3-6... etc, all the way down the neck. As you move down the neck and the frets get closer together, use the shorter edge of the credit card as your straightedge... you want to span just three frets. If I know which string(s) are buzzing, I will test this way all down the neck right next to the buzzing string... if not, I'll check near the middle and each edge of the fretboard. Again... your tech/luthier could find a high fret for you, but I like to know where to have mine look for when I take my guitar in. Leveling/dressing a high fret is definitely NOT something I do myself.
I think that's enough to start with... hope it helps!
D
