The reason parks are never hunted out in terms of old coins is that the average metal detectorists does not swing slow, does not use small coil, does not clean out all the junk in the hole, and does not dig deep deep targets. When the average metal detectorists hunt in a real trashy area, they find the display jumping around like crazy as well as being overwhelmed by all the many tones.
Set your AT Pro set on Pro Zero (other detectors in the all metal mode), iron audio on, notch out 65 and below. Swings slowly to tease out the high tones. Do not rely on the display numbers. Notching out 65 and below will eliminate the display and sounds of all the pull tabs and foil and other undesirable junk. We are looking for coins, not gold rings.
You should dig out all the trash out of the hole to eventually get just the high tone target.
If you are getting a high tone, there is a high tone target mixed in with junk. You cannot go by the display, since it jumps all over the place. The same goes for the tones, they jump all over the place. This is why you notch out 65 and below, to eliminate some of the distracting noise. If you hear a high tone, there is a high tone target in the hole. Once you think you have removed all the junk, swing your detector back over the hole to see if you are still getting a high tone. A lot of coins are found at the 4-6″ level, but the deeper signals increase your chance of finding a coin since the average park hunter does not dig that deep or clean out the junk. Notice that when you clean out the junk and pass your detector coil back over the hole, the high tone will be a lot crisper. When checking the depth of a target in a hole with junk or other targets, they can throw off the depth indication reading in the display. The AT Pro depth indicator tends to be more accurate on a single target. I rarely look at the depth display, but listen to the relative volume of the high tone.
Go to the park on the weekend and just observer where the people are located. In my park along the river, I see people playing volleyball, families having their picnic lunches at the concrete tables. The most desirable tables are along the river under the trees. The people are wading in the shallow part of the river near the rocky shoals close to the river bank. There is one trail up river where the majority of the fishing takes place and beside that a youth soccer field that is in use on the weekend virtually all day. So these are the areas of the park that I hunt once or twice each week since it is within 5 minutes of my home and I need to take a break from writing and do some swinging.