Rattle whilst idle, controlled by lights.
 

Hello all, its been a long time since I visited.
Question (hope I'm in the right forum), had to use a drive-thru the other night and whilst sat there wasting my fuel, I decided to turn off my headlights and discovered I had a rattle which disappeared when I turned the lights off and came back when I turned the lights on! Its a Peugeot Partner 600D Box van. Is something underpowered? Don't remember my old van doing this, which is 6 years older, a Vauxhall Combo Box van. The Peugeot cranks a couple of times, and the engine rocks back n forth before it starts. My Combo instantly fires like a new vehicle!
Is my battery failing?

Thanks

Mike, over here in England.

Does the idle RPM drop a little with the lights on? It could just be something that vibrates at a particular RPM.

My headlights make a distinct fan noise when I turn them on. Maybe a rattly fan?

Just thinking out loud: faulty alternator (bearings, clutch, belt, regulator) or a faulty idle up (rpms dropping too low under load).

The van was idling high, and drinking lots of fuel. The van belongs to my boss. I solely use it. I constantly nagged and he told me to get the mechanic out and connect to OBD11. Turns out the advance solenoid isn't working, so the engine doesn't know its warmed up! The mechanic manually lowered the RPM. A couple of you mentioned RPM, and reminded me of what he's done. Its hovering around 0deg Centigrade around these parts, so heater's on, lights, not so powerful radio. The boss wants to upgrade to an LPG in part exchange, guess I'm stuck for now? If its idling too low, battery won't charge properly?

Thanks Mike

Unless its too low of an idle speed (sub 800 rpm) or it's an ancient alternator design, it should charge just fine. Modern alternators can put out almost full power at idle.

BTW, your rattle is probably due to the engine running at exactly the right (or wrong) speed and load to cause the engine to vibrate a bit, making other loose things rattle (heat shields). Could also indicate a broken engine mount. At any other RPM or load (like with the lights OFF), it won't do it.