It sounds like cam followers rattling, caused by neglect of an oil change or the use of poor quality oil resulting in worn cam and followers or, if it has the hydraulic cam followers, after the engine has been stopped for some time, these have leaked down (which they do any way) but lack of good oil or regular oil changes causes them to stick down (gummy deposits left internally) until the engine is revved which then increases oil pressure and, if you are lucky, they become unstuck pumping up, and engine then quietens. Has it had a head gasket fail? Antifreeze can stick those hydraulic lifters but any of the above apply change the engine oil and filter ASP to be on the safe side, its a cheap fix if its not to late.
Sounds on the computer speakers don't replicate well and with the bonnet down the exact spot from where the noise is coming from could be misleading. I have had people bring cars with (supposedly) cam noises to be repaired and have found the real problem to be something else such as a broken clutch plate, a loose belt pulley on the crank or an air con pump about to fall to bits.
