Strange sound after fixing radiator

[AirSpeed]

I pulled the radiator in my 72 GMC 4X4 350/350 yesterday to fix a leak, this morning I drove it and its making a really strange sound. It doesn't do it until I put it in drive and give it a little gas, it sounds like "tink ta tink tink tink" (that's the only way I can describe it) it doesn't seem to do it in reverse, only drive. It doesn't seem to increase with speed, just makes the same sound. I feel no vibrations, it goes down the road smoothly like always. I can't see anything hitting under the hood. If I hold the brake while in drive it makes the same sound, even when the trucks not moving. Any ideas?
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