unstable / distorted / glitchy digital line signal
If you think that MISO may be picking up interference or energy from the RFM43, you could try adding a small capacitor (try 50pF, 100pf, 470pF, 1nF) to ground and test again. If the glitches go away then you probably are experiencing EMI or Electro-Magnetic Interference. 

To verify this, remove the capacitor and construct an aluminum-foil "dome" around everything except the RFM43, careful not to touch any bare metal. Put an alligator clip on the dome and connect it to ground. Power it on and check MISO - if it's clean, then the dome is blocking RF interference from coupling into the MISO line. At this point you need to work more on how your unit is designed and shield these lines more from the RFM43.

If it does nothing, then your issue is elsewhere. Reduce the SPI clock rate; high-speed-anything always complicates things. Also, long connections between Arduino and RFM43 would also make interference more likely - make sure all wires are very short. Also try adding extra bypass capacitors near each device and see if that helps. Also read the datasheets carefully to ensure you have the correct lines going to the correct pins. The RFM series are notorious for having tiny little details buried in the datasheets.