I think the Miata is broken... There's some rattling/tapping coming from what sounds like the front of the head. It burned oil something fierce when I stuck the Royal Purple in (which is now gone) but this sounds serious, metal-on-metal.
Common first-to-go parts? Cam follower/rocker? Valve? PISTON?
does it sound like either a spring rattling around on a shaft, or the sound of sucking air through a straw at the bottom of the cup when you put your foot down a bit in 4th or 5th gear? if so, mine is doing that too, started just after I did an oil change even though oil pressure seems just fine and everything is properly tightened down
just for giggles, make sure it's not the REALLY obvious answer that something is in fact rattling around due to the bolts either not being tightened down properly or having backed off due to vibration
No, this is metal-on-metal and internal. It increases with RPM.
In reply to Javelin:
If that happend to me and I did something eariler like, say... replaced the timing belt and water pump...I'd open the front up as a first thing to check my work.
I did see a Miata engine in our neighborhood on CL....
In reply to jeffp:
But the sudden increase in oil is a sad-sad symptom....
I checked all the outer stuff on the nose of the motor, nothing loose, nothing weird. I almost want to pull the valve cover for a looky-loo. We'll see what kind of noises it's making tomorrow, I might drive it down to the Street Survival School. It has over 2,000 miles on it already since the water pump/timing belt as well (remember, we drive it every day 
).
Just pull the valve cover and see what it is. What is the hesitation?
A squeal at mid engine height would indicate a timing belt idler pulley failure.
A rattle a the top of the engine would indicate a loose timing gear.
Note: Miata's do not have rockers. The cam acts directly on the lifter which acts directly onthe valve stem.
Broken crankshaft snout?
No noise today. Weird car. 
Sounds like a HLA, hydraulic lifter adjuster problem... Common Miata problem.
Although, I would pull the oil filter and cut it open and see if you find metal.
http://www.miata.net/garage/hla/index.html
http://www.miata.net/garage/hla/cleaning.html
I thought this was the problem with the miata I just bought (just before I bought it), but it wasnt this.
Good idea Apex, it's oil change time anyways...
Javelin wrote: Good idea Apex, it's oil change time anyways...
Miatas often let you know its time to change the oil by making a little noise. Synthetic sometimes helps.
Mine always makes a little noise on startup. I run 5W30 synthetic. It did just roll over 162,000 though...
The story with mine is that while I was negotiating to buy it the motor had a tick that I thought was an HLA issue. The PO took me for a ride-along and the tick suddenly got worse halfway through the run. First thoughts were collapsed lifter, cutting the filter showed metal though. PO put in new motor before I purchased it. (miata motors arent that expensive from a Junkyard.)
I did a lot of reading on the HLA issues, that IS the ticking that goes away after startup on all miatas and why they might tick a little after a hard auto-x run. At least according to what I read, it makes sense though.
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