I too have a clicking in the rear end, it has gotted worce. A scrssching noise now heard. Walking nent to it has not helped, appears to be internal.

There are two hexagonal bolts on each axle on the back wheels that are rubbing against the frame of the bike. Take the back tire off (or just loosen the outer bolts and move the wheel away from the bike a bit. Then loosen those two hex bolts so they aren't rubbing on the inner frame. You might need two wrenches, one to hold one of the bolts and the other to turn the 2nd bolt.

I have been through two Schwinn 26" trikes. A Meridian and a Town & Country. They both have the heavy clicking sound in the rear end.
My Schwinn dealer finally discovered the mysterious problem. Schwinn help line group couldn't find or did not want to admit the problem.
The end of the drive axle's flat side is rounding off as time marches on and thus click click as it wears down. Schwinn repalced one rear axle to fix the problem but alas it also wore down. They wore down within 40 miles usuage. Faulty batch of rear end axles. Hm-m-m-m.
Now I want a replacement Miami Sun trike for my trouble.

If your rear end is clicking I should see a doctor - it might be something serious.

It could be one of many things,The first Thing I would do is check whereabouts the noise is actually coming from,ie lifting the back of the bike up with a help of a friend and spinning the wheels one at a time and after both wheels being spun rotate the pedals and see if its coming from the gear cog at the rear,Whatever is the problem the part needs to be cleaned out and a new part put back on.It may be just a cracked ball race in one of the wheel spindles.