I've got kind of a rattling/grinding noise happening only when under full vacuum (like off-throttle engine braking). It's a pretty fast reciprocating sound. As soon as I give it a touch of gas it will go away, but if I let off completely the rattling comes back. I'm wondering if it's the valve guides giving way and letting the valves rattle around under high vacuum; just a thought. I overheated a couple months ago and blew my head gasket; I haven't pulled the head yet unfortunately, but it's the next thing on my list.

When I do I'll obviously be re-surfacing the head (and any milling that might be required), new head gasket, new valve seals and will probably get a 3-angle valve job while it's out. Just wondering if you guys think I'll need new guides too. I don't have a lot of extra cash right now, so if I don't have to do the guides, I won't, but can't figure out what else this noise might be. Would bent valves do this? Springs? Retainers?

And also for info, under engine-braking my ATTS light will come on and give me a PGM-FI communication error. My guess is the ATTS control unit is not seeing the numbers it's expecting from the MAP sensor if perhaps I have a leak problem with my valves.