Weird sound as revs drop (Page 1 of 2)

Only you can investigate to see what the sound is and where it's coming from.
  Apparently this is repeatable so it should be something you can listen to..
  Let's start out with facts first shall we?
If it's a noise it's not going to fix itself.. in all likelyhood it will cause as much problem as possible during the race and that includes the things you are afraid of.
So let's identify it and see what can be done or what should be done..
  I have a stethscope. Something I paid a dollar for in one of those tool bins decades ago..
Prior to that I would use a long screwdriver, a rod, a long socket extension, anything to conduct the sound from where it was occuring to where I could hear it... Just prior to getting the stethscope I had a long (about 4 foot) brass rod that on the listening end I bent into a tight loop.  The rod was about 1/8th inch thick brass welding rod.. Could have been almost anything..   Put one end of the metal rod on the suspect area and the other end put right against your ear.. If you have to bend it  to get it into the suspect area it's OK! straighten it when you are done, (or not!)
  Now start out with a known good engine.., Your daily driver for example.  Start at the accessories first (they're easist to deal with) Put your rod right on the alternator.. (carefull to keep hands fingers head etc. clear of moving parts but you can slide the rod etc. into areas close to moving bits if you take great care.
Your alternator if it's in good shape should have a whirring sound.. no Squaling no grinding just a steady whirring sound.. OK go listen to the race car's alternator..
  if you hear a similar sound well you can be reasonably sure the alternators OK
   Now slide the rod or whatever as close to the nose of the water pump, watch out for fan belt etc.. but you should be able to do it..  Listen carefully and you should hear a sloshing but also a whirring sound..  That's good.. Now switch to the suspect car.. if the water pump sounds the same you again in luck, grinding knocking etc.. means it's time for a new water pump.
  I don't know how to tell you about rubber timing belts,  I've no experiance.. Timing chains on the other hand are best listened to when the engine is cold upon start up.   Stick your pickup down near the crankshaft on the timing cover.. You can actually hear the chain.  any snatching, slapping,  or knocking and it's time to replace.. Listen down at the crank and up at the camsghaft/s.  The most telling will be the area between the crank and camshaft.. it will be running on a guide and it's at it's worst there..
  Now stick the rod on the bottom of the engine block about 2-3 inches above the pan and listen.. the crank will be smooth and quiet, rumbling is a precursor to the bottom end dropping out  any knocking is likely to be rod knock with one cylinder likely noisier than the rest..
Check all the main bearings and all the crank throws.. (basically line up with the sparkplug down on the engine block)
OK slide the rod up to the camshaft/s Ticking is perfect.. clattering might be OK if not too loud and any knocking etc.. is cause for immediate attention..
  Check the oil pump.  You can hear it's whirring and sucking sounds..if in good shape.. Then slide the rod up around the base of the distributor. 
  OK if you have fuel injectors.. check each one of  those too.  a sharp clean clicking, ticking, slapping, sound is good..  A softer sound indicates a dirty injector.. no sound means the injector is completely plugged..
  Disable the ignition and listen to the starter.. the solenoid if in good shape will be that slight click just before the starter cranks.. a zap indicates it's time for a new solenoid..    the starter should crank over at a steady pace assuming the battery is fully charged and the starter isn't worn out. 
   So know, tell us what the sound is.. or at least exactly where it's coming from..

Good write up for us all and agree the use of a stethascope is a great way of getting used to your engine sounds when it all works and then listening for issues just as described above - thank you!

Is it a grinding, or a clattering?

It's a 7M, so chose from the following.
A.Blown Head gasket (Not necessarily the cause of the sound, but hey)
B.Bent rod from previous BHG hydrolocking incident
C.Piston slap caused by cylinder corrosion from previous BHG
D.High mileage rod bearings

I would have said a rod bearing knocks horribly rather than grinds, especially on feathering rather than over run.....it could be old seals complaining about having to work or pully bearings at a guess!

This thread is pointless without video. :-/

What a great bit of information!
   thank you!
   I'll pass that along..
  a stethscope makes diagonsing an engine really a piece of cake..   just put a mental image of the internals in your mind and put the scope up to that spot and listen.. your engine will tell you.. If you aren't sure listen to that spot on a known good engine..

are you sure its coming from the engine and not the trans or torque converter? You didn't mention whether or not it was an automatic or manual.

What those guys suggested with a stethescope is great I also used to use a piece of fuel line to my ear. Just be careful you dont touch something moving while you do it or you wont be able to hear for a while.

Piece of 1/4" fuel hose held up to one ear, with an earplug in the other, makes a pretty good stethoscope, btw.

I'm decidedly not my team's mechanic but I like to hear myself talk, so - if it's on deceleration it sounds like something is loose in its bearing, so when it is under power it gets held in position but then vibrates around when it freewheels. Could it be the transmission input shaft maybe? Does it still do it with the clutch disengaged?

Loose rod bearings will definately have one bearing sound louder than the others..  In fact it's not unusual for only one or two rod bearings to be bad  Listening to the block will identify the differance between bearing sounds caused by mains or rods..   Line up with each spark plug and then listen an inch or two above crank cenerline..