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Gently rocking the bike back and forth with front brake applied - slight knocking sound and movement? Headset isn't loose and forks aren't extending/compressing.
What fork?
If you have disk brakes check its not the pads moveing
Vanilla
turn the wheel 90degrees and try again (that should isolate any pad movement)
Try storing the bike upside down for a while to let the bushes in the top of soak some oil up.
if you wrap your hand around the seal on one leg and rock again you'll feel the movement there if it's the bushes
Thanks all
May just be the pads moving in the caliper. Pull the brake lever on as hard as possible, to stop any pad movement, and try again.
Also check your front wheel bearings, brake caliper mounting bolts and rear shock bushes.