Your hydraulic theory is correct. Earlier 15X sprinters used a mechanically driven alternator for electrical power, but the 158 and 159 use a hydraulically driven one, thus you hear the whine start when the load on said alternator is increased as the interior lights come on at 47 seconds. :)

@@doshtechvideos You've just answered a 30 year mystery for me. Its been driving me mad working out what the hydraulic noise was. I went on 158's many times from Salisbury. Often they were off and then started up the cummins lorry engine with the usual power steering humm then they add in the hydraulic cat meow soon after. I kept thinking it was gear engagement of the voith box because of the ZF buses I used to drive. ZF Volvo buses used to groan during a gear engagement. And when Sprinters came straight off the power. The hydraulic whine would make a calming down settling noise. Like a settling transmission. Then I was confused as to why the lights all came on suddenly as soon as the hydraulic cat meow started. Now I know! You learn something new each day. Thanks for clearing that up!

​@@doshtechvideos On our 158s is between 60/90 seconds for some strange reason