Help with strange knock???

There is a vacuum hose that connects the brake booster to the intake manifold. It could be when the brakes are applied the booster leaks. The loss of vacuum may result in an engine stumble, and related noise.

Plugging to hose for test will result in heavy brake pedal to stop, so be careful.

The problem is likely two fold, what you feel is related to stumble, but something is also too close or loose too.