If you receive a weak, distorted signal close to the maximum usable frequency, what type of propagation is probably occurring?

When operating very close to the Maximum Usable Frequency (MUF), normal ionospheric refraction begins to fail, and the primary signal escapes into space. However, small irregularities in the ionosphere can still scatter a tiny fraction of the wave's energy back to Earth. Because this scattered energy takes multiple, slightly different paths to reach the receiver, the radio waves arrive out of phase. This multipath arrival creates a characteristic weak, fluttery, and distorted sound. Ducting and line-of-sight are unrelated to the MUF, which is strictly a metric for ionospheric refraction. Ground waves are unaffected by the MUF and would generally produce a clear, though short-ranged, audio signal.