If you're hearing an echo that is not being streamed or recorded, then you probably have monitoring turned on for a source that doesn't need it.
Check:
Edit > Advanced Audio Properties
For sources set to "monitor only" or "monitor and output".
If turning that off works, great!
If turning that off means audio doesn't make it to the stream or recordings, that means your setup was really broken from the beginning and you need to start over.
Usual setup is for OBS to be set to capture your default Windows output, and then for anything you can't hear that you need to hear, like a capture card, you set up an audio monitoring device in Settings > Audio and then turn on monitoring for the sources you need in Edit > Advanced Audio Properties (Mixer).
The problem is that you can't use the Windows default output for monitoring if you're also capturing it.
