Is there any place you can put in a coin garden? Place some common coins ie, copper penny, dime, nickel, quarter, in an area where this no other signal and just a solid threshold hum. Get them in the ground 6 inches or so (separate from each other) and practice listening, and pin pointing. I'd do this first.
To be honest with you, to me it sounds like several things could be happening.
1. You running in Quickmask and All Metal
2. You running in manual sensitivity and have it really high or maxed out.
3. You might have some EMI problem. If you have a spot where this a large buried electrical cable, sometimes that can be a problem. I've only had it happen to me a couple of times one was at a football field and the other was a fairground.
Another suggestion is to get a copy of Andy Sabisch's book on the Minelab Explorer and E-Trac handbook.
There are a number of videos on Youtube. Click on my link below and take a look. I've also subscribed to a number of other E-Trac users (Vidslayer, Neswiper, Goes4Ever, OhioCoinHunter,). These guys do some FULL digs as well and I think they're helpful to watch.
Main thing is that coin garden and see what's going on. Feel free to ask more questions...great group; always willing to help.
NebTrac