Weird Engine Sound

[ScoobySteve]

So I've noticed a bit of a bizarre engine sound.

It's very hard to describe the sound except to say it's a "wishy-washy" with a some lower frequency knock. Here's the weird thing. It's not really there at idle, only when revved between 1000 and 1500. The sound is the same under load (in reverse at low speed) or just in neutral. While driving the motor is smooth and no sound (at least that I can hear).

I'm hoping it's just the water pump as I'm doing a top / front engine service soon. I searched the threads as obviously the big fear is rod bearings. But, my oil pressure is great (5 bar at idle when cold, just under 3 when hot). Also, I don't notice anything when driving at speed.