Lean engine squeal when accelerating

[kpassaur]

I just purchsed a used 05 f-250 4x4 with a Power Stroke 6.0 engine. I am new to diesels (well I had an Olds in the 80's but that should not count). I have noticed that it makes numerous noises which I assume are all related to diesels except for perhaps one.

When I first start it (starts right up) and run it the first mile it is sort of doggy, after a mile and it is warmed up it runs very well. However, once warmed up it makes a noise when you accelerate after coasting. In other words if the engine is always accelerating or providing power it does not make this noise. If it is cold it does not make this noise. The noise is very brief and sounds like the noise cars used to sometimes make back in the eighties when they were running lean. Sort of like a little squeal (not very loud) is this normal? I really cannot tell if it is coming from the transmission or the engine.

Any idea as to what it would be or if it is normal?