I'm no expert but I do own the MX5 and will try to offer what scant advice that I can.
1. Call Whites. I wouldn't rely solely upon an email before I decided the machine was no good. If it is not functioning as it should, Whites will repair it for you (the warranty is two years from date of new purchase).
2. Whites has a forum (google - whites electronics forum) with a lot of very knowledgeable people. Someone, or more likely - several someones - will be able to help you out.
The MX5 is a busy machine; meaning it does talk quite a bit. I like that because it lets me know what is going on under the coil. I always hunt with 8 tones when using discrimination, and sometimes the machine will give a brief high tone over something that has been discriminated out. It doesn't sound quite right though, but does give me reason to investigate that spot a bit more. From there I decide whether I wish to dig or not. On many occasions I've found that the high tone was a good target - mixed in with junk. I'm always a bit amazed when I find a good target surrounded by junk.
I don't use two tone, but I believe the high tone begins (in that mode) with the first accepted VDI.
I frequently hunt in all metal mode (single tone) - provided the area is NOT full of trash. When a potential target is located, I take note of the VDI and switch to discriminate to see if I get a high tone with a similar VDI. The all metal mode helps me figure out the rough size of the target and (supposedly) goes deeper. I can't verify the deeper part, but have no reason to doubt it.
Ground balance makes a big difference. I know the machine has auto ground balance, but it really helps if you get a good ground balance at the start.
I believe the stock threshold setting is 20. For most places I've hunted, that is too high. I sometimes go as low as 5 or 6 on the threshold setting and almost never above 14. I have no idea if the threshold is part of the problem you're having, but it certainly makes my MX5 chatter constantly if I have it set too high.
I run my sensitivity about two notches below the top level most places - just enough so the threshold is barely heard in the background while I'm detecting.
I think the deepest good target that I've found was right at 11 - 12 inches. I've dug quite a few quarters and nickels that were around 10 inches. I think my deepest dime (or copper penny) was in the 8 inch area. I do dig zincs, but don't believe I've ever dug one that was more than 4-5 inches down. Just today I dug a quarter that was 8 inches down. I thought it was silver because the VDI was 84-86 (usually 83 on a clad quarter). It wasn't silver but I'll take all the quarters I can find.
Sweep speed also makes a difference. I've noticed that on those occasions when I get that off-sounding high tone and go back for a second look, I sweep a little slower to really investigate what the machine is telling me. When it's junk, the slower sweep speed never gives me that brief, weird, blip of a high tone.
It is entirely possible that the MX5 is not the machine for you. There is nothing wrong with that, if that is the case. It seems that most people around here hunt with a Minelab (either Etrac or CTX 3030). They're fine machines (I had both) but they're not for me; I just couldn't jell with them in the way that I have with the V3i and the MX5.
Detecting is a lot more fun when you're at peace with your machine.
