E38 740 Engine noise ticking / tapping

Hi My 2000 740 has a ticking / tapping on the engine.
At idle, the tick is around 3 times per second, and is definately linked to the number of revs.
The sound is not very loud, but definately audible with the bonnet open, and when going down corridors etc. It is also audible with the radio off, and window down, from within the car.
The car has 95,000 miles on the clock, and 'was' under BMW warranty when I took it to my local dealer 6 weeks ago. They advised me to carry on driving the car, as the noise did not have any affect on how the car ran. However, as the car was under warranty, I wanted it to be addressed, before something does go wrong which would cost me money.
The dealership are 'clueless' with regards to the problem, and have said that unless they can find a problem that warranty would cover, I would have to pay for full investigation, and probably for the repair. This seemed strange, as it was one of the reasons which I wanted a warranty in the first place!!
Anyhow, I have started to try to investigate the problem myself, even though I'm no mechanic. I have borrowed a stethoscope from a friend, and held it over the rocker covers of each side of the V8, expecting to be able to hear a loud tappet or valve lifter. There appears to be just an 'echo' of the tick in these areas. However, if I listen over the case ends (over the timing chain covers), the tick is very much louder. I'm not sure if this would be a genuine timing chain issue, or something else in this area of the engine. I've had 3 740s and a 745. 2 of the 740's had mileage in excess of 95000, (one with 130,000), and both were lovely, with no engine noise. My limited mechanical knowledge would be suprised if it were a chain issue, as I would expect a more constant 'dragging' sound, rather than a tick. (??)
Does anyone know how many chains are on this car?? My dealer tells me there are 3... (?) One for each side of the V, and one across the 2 sides.
Has anyone come across anything like this before?? Any ideas on what it may be, or what to check?? Any ideas on likely repair costs and 'seriousness' of the problem would help also.
Many thanks

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