Flutter echo is a rapid, metallic repeat you hear when you clap between two parallel hard surfaces. brick walls are especially prone because they are almost perfectly reflective.
The fix
- Clap between the two walls and listen for the buzz.
- Put one panel on each wall at the height where you clapped, offset by a panel width.
- Clap again. Add a second pair if any buzz remains.
Two to four panels usually solve it. Diffusers or angled surfaces also work. See flutter echo explained.