Truck make a noise I dont like -HELP-

[PHOENIX]

My truck has been making this noise lately.
A few weeks now. Doesnt seem to be getting worse.
It only does it when accelerating.
Not very noticeable from a stop, but when you go to second gear it can be heard.
I have a manual transmission.
I do not really hear it from second gear to third.
When a vehicles timing is off and you turn it off you get that brrrrrr sound.
It kind of sound like that brrrrr, but not as loud.
I notice the sound most when I am turning a corner and do not down shift into first.
If I keep it in second and start to hit the gas it makes the brrrr sound the loudest.
My truck has had an oil leak for awhile and it did get below the add line once recently.
But it was making the sound awhile before that and doesn’t seem to have gotten worse.
I do not know what this is, is this a valve sound?
I am going to check my timing and a few other things later today hopefully.
Could a sloppy transmission cause this sound? My trans is rather old and has been through a lot.
It is a little sloppy and sometimes clunks when I put it in gear.
The sound seems like it comes from the engine bay, but its hard to tell.
It only makes the sound when driving (under load).
Truck seems to run fine and strong (well as strong as a stock 350 could run).

What do you guys think this is?
What should I check to help figure this out?
Thanks