I have a 2010 extended 3500 Express van 6.0 with 300300 miles. Engine has always been strong, I change oil regularly and switched to full synthetic 5000miles ago. I have heard a slight ring/knock like a bell. It was not loud and I have a front end issue that I assumed is were it was coming from. Yesterday after driving at least 100 miles as I pulled onto my road it got louder and sounded more like a knock coming from under the van. it was not as loud at hi rpm but still there. It sounded like from the engine/transmission area. No signs of leaks, oil & transmission fluid were normal. From under the hood the noise sounded not as loud but very clear. I checked the fan clutch & water pump both seemed ok. after running at idle for 30min while looking it over I noticed the oil pressure down to about 10psi. it usually is always between 30 - 40psi. I changed the oil with conventional oil and the pressure jumped back to 38psi. but knock still there, it sounded different and with a steering wheel vibration. I am not great at diagnostics like this. Any suggestions would help. I am new to this forum thing so please understand. Thank you

 

Does it get louder when the A/C is on? (at idle)
Flexplate may be cracked.

 

A/C is disconnected. It seems to get quieter at acceleration.

 

My wife's Cadillac had similar symptoms. Noise was louder when the A/C was on. Cadillac tech replaced the compressor and was surprised when the noise was still there. Thats when the 'Old Guy' mechanic had him check the flex plate. It was cracked across the crankshaft mounting bolts. Evidently the load from the A/C compressor put enough stress on the engine to cause the crack to knock as it flexed back and forth. Noise appeared to come from transmission and the engine up front but only because it had transmitted up the crankshaft.
New Flexplate and all was well..
An easy check to do before pulling the engine apart.

 

Rod knock will rattle when you let off the gas

So will a loose U-joint

I'd check the flexplate first

 

Is there a way to check the flexplate without removing the transmission?

 

remove the cover