I have a similar slow "ticking" that is audible and transmitted through my steering. It is only when the A/C is on, and felt only when idling. It occurs when my thermostat is set on less than maximum cold, and happens on a cold day when the A/C compressor kicks off. I have located it to a ?solenoid switch which regulates the heating. When the thermostat is activated, it switches the A/C compressor and activates this switch which then causes this clicking sound. Being in the tropics, my heating system has been disabled (hot water inflow and outflow blocked-up and excluded), a common practise here.

However, I am not sure if this 126 system is applicable to your model, although the 5-sec interval sounds remarkably similar. I hope this helps.

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1983 w126 280SE - daily drive
1983 w123 200 - sold