HI Thelady...

Welcome to AMHD. What makes you sure your shower doesn't have a PTRAP... please give us more details here?

For example, is this a basement shower? Is there standing water in the shower drain? How old is the shower? Tile or fiberglass/acrylic shower? The more info. The better the answer we can give you, OK?

Mark

House built in 1853, basement kitchen, toilet & shower. Shower has an iron drain with a inverted cup covering with a little indenture which lets the water through. All drains make noise when another is in use. Shower drain goes to street sewer line

You probably have what is called a house trap. That is just one trap for the whole house instead of individual traps for each fixture. That's how they used to do it way back when, until they figured out they just don't work all that great. Of course code now calls for a trap on each fixture, but your grandfathered in. Are you experiencing sewer smells? Not much you can do without breaking up the floor and adding traps and correct venting. This would be a large undertaking. Im not even sure they would have used a house trap back in the 1800s, but hopefully they did. If it were my home, I would seriously think about a total plumbing renovation. Good luck.