Mine does this too, started a few weeks back, getting more persistant since then.
Reach down to the ps pump pulley, grasp it and pull forward then backward. Pay attention to whether just the pulley moves, if so just tighten nut as previously suggested - if shaft and pulley move together then you have internal ps pump problem (as is the case with the Bomber)
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"Bomber" '67 El Camino
Beater comes back to life
350/TH350 14.90 @ 93mph!