Seeking 1970s YA mystery. Protagonist delivers newspapers and in a subscription sales contest. Has a genius inventor friend (including retractable staircase). Mysterious house with strange noises. Turns out to be a family of circus performers who buy a lot of newspaper subscriptions.
I’m not completely sure, although it sounds like it might be part of certain boys’ mystery series from around that time or in the 1960s, like:
The Marvelous Inventions of Alvin Fernald
https://www.goodreads.com/book/show/1039249.The_Marvelous_Inventions_of_Alvin_Fernald
The Brains Benton Mysteries
https://www.goodreads.com/book/show/2311878.The_Case_of_the_Missing_Message
The Mad Scientists’ Club
https://www.goodreads.com/book/show/222127.The_Mad_Scientists_Club
Hope that helps!
Brains Benton – Case of the Missing Message