Timing circuit with NOR gates

[Jester]

Can someone explain how the diode-resistor portion of this circuit works?

Note:

1) The schematic was determined by “beeping out” the traces on an existing PCB so there is a chance I missed something.

2) I used a 100:1 probe for the D1 signal to minimize loading.

 

[FvM]

there is a chance I missed something

You obviously did. The circuit can't work with floating gate input. Try:

 

[Audioguru]

Here is a normal Cmos oscillator circuit: