I was beginning to suspect that.
4 seconds sound clip from the After the Thin Man (1936) movie soundboard.
You can hear this line at 00:58:56 in the DVD version of the movie.
Quote context
[...]
- At least he might give his version as to why Dancer threw him out.
- If that little rat tries to hang anything on me, I'll...
- You leave my brother out of this!
- I was beginning to suspect that.
- No, not yet, but you ought to stick around. Outside. It's, uh... Very close in here.
- Where is this brother of yours that didn't kill anyone?
- He lives in a hotel on Turk street. I don't know just where.
- You don't know much of anything, do you?
[...]