The microwave started making a very loud noise when the magnetron was firing and it wasn't heating food. My understanding is that this means either the magnetron or the diode is bad?

The diode physically looks ok - there are no signs of being burnt or other physical damage. I tried testing the diode in the following manner:
1) Connected the anode of the diode to the positive terminal of a 9 volt battery (the battery measured 9.25 volts).
2) Connected the positive probe of a digital multimeter to the cathode of the diode.
3) Connected the negative probe of the digital multimeter to the negative terminal of the battery.

The multimeter showed 4.25 volts. Is that an acceptable voltage drop, or should it have dropped more or less?

I tested the magnetron by putting my multimeter on the lowest (200) ohm setting, placing 1 probe on 1 terminal of the magnetron and the other probe on the other terminal - the reading was 0.2 ohms. My understanding is that it should be around 1? I also connected 1 probe to 1 terminal and the other probe to the case of the magnetron - the ohm setting was 2000k - and it showed infinite.

Both components were tested out of circuit.

I would like to replace the defective part(s). Do these results indicate the magnetron is bad? Is the diode bad too? If I'm going to the trouble of replacing the magnetron (assuming it is bad), should I just go ahead and replace the diode too? Thanks.

p.s. By the way, I discharged the capacitor before doing anything (no spark) and I tested the resistance of the capacitor out of circuit (2000k ohm setting) and it ramped up through the numbers and quickly went to infinite.