Trying to find two noises in my 3ltr petrol 407 coupe, cold start rattle and rattle under acceleration especially from a slightly higher gear before kick down occurs. note diagnostics show anything.

All the things you mention at the end would cause codes on a scan tool so it is unlikely to be those.

A high end $5 grand scan tool computer should diagnose the acceleration issue, but they can't diagnose bearing or gears that are worn.

You could try cleaning the Throttle Body and MAF.

peugeot 407 v6 3.0 very short-time rattling(grinding) noise at ...

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https://www.youtube.com/watch?v=ZRJzci7qvNA
Feb 27, 2012 - Uploaded by reza679
peugeot 407 v6 3.0 very short-time rattling(grinding) noise at cold-start, source ... but a friend told me it can be anything else as well (like those solenoids)! ... C6 makes when starting cold - is that a VVT or a valve lifter noise?.

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Nissan had a problem with the exhaust bracket being cracked or broken

A KNOCKING SOUND IN THE ENGINE IS MOST LIKELY CAUSED BY ONE OF THE FOLLOWING
1. PISTON SLAP: Makes a sharp metallic noise. Idle engine and short out each cylinder plug. The noise will disappear when plug with bad piston is shorted. Noise will also disappear at acceleration. This can be caused by worn or out of round cylinder, or broken piston ring. Correct problem by re-boring cylinder and/or replacing piston.
2. VALVE NOISE: Makes clicking or rattle noise. Caused by excessive wear on valve stem or lifter, out of adjustment, or stuck valve. Correct by adjusting valve clearance, replace worn valve or lifter, regrind cam, replace valve guide and /or valve. A stuck valve can sometimes be loosened by passing oil through the carburetor while engine is running.

3. ROD BEARING KNOCK: Makes sharp metallic noise similar to a piston slap. Detection is opposite of piston slap. Rod knock is not heard at idle. Knock becomes louder as engine speed is increased. Caused by excessive rod bearing clearance. Correct by adjusting rod bearing clearance to .0015 inches by removing shims. May require re-pouring rod bearing.
4. REAR MAIN BEARING KNOCK: Makes dull knocking or thud noise. Detected at speeds between 20 and 50 MPH. Knock will normally decrease or disappear while pulling or decelerating. Noise will be detected the loudest at normal driving speed, when not pulling or decelerating. Correct by adjusting bearing clearance to .001 to .0015 inches. If knock is excessive, crank should be checked for out of roundness. May need to re-pour all main bearings to correct.
5. TIMING GEAR KNOCK: usually the most difficult to diagnose. If gear is loose or badly worn it will knock in all ranges. Run engine slightly above idle speed. Slowly open and close throttle. Knock will continue to be present, but just as engine slows down knock will become a slight rattle. Remove timing pin and reinsert into timing hole on timing gear cover. Press timing pin tightly against timing gear and accelerate slightly above idle. Knock will significantly be reduced or disappear. Correct by replacing both timing gear and crank gear as a matched set. The two gears should have a backlash clearance of .003 to .004. If more than .009 inch backlash, an oversize (.005) timing gear should be installed.
6. WRIST PIN SLAP: This can not be detected by shorting out the cylinder plugs. Rapidly accelerate and decelerate the engine speed. The engine will pass through a certain speed range when the wrist pin will rattle at about the same pitch as a valve tappet noise. This can be corrected by installing a new wrist pin bushing in the rod or new wrist pin f badly worn. Wrist pin should fit the piston and connecting rod with a tight metal to metal fit. The pin can be pushed into the piston and rod with a slight pressure of the hand. Pin to rod clearance is .0003 to .0005 inches.

try U joints or check all you heat shielding on your exhaust it might be harmonic. My 96 300zx rattles alot when i give it some gas and it sounds like tranny but the trans seems to be smooth for the most part and the clutch is relatively new. also check your trans mount. i've seen bolts get loose and it wont knock but it will rattle.

Could you give a little more info?
IE: Car make/model, age, diesel, petrol etc??

If the car is making strange noises whilst in 1st gearm then this could be a sign of wear on the gearbox.
I would suggest topping up your transmission fluid (either into the gear-box itself, or ctop up normal engine oil).

Unless you are hearing an exhaust rattle of some sort, suggest higher octane fuel or check your timing.

Your clutch is goin g out on it.

Check the heat shields on the exhaust system. They get loose from the extreme heat fluctuations work loose and rattle. Especially the one between the engine and firewall where the 02 sensor screws in. It probably rattles less after warm up because heat causes expansion which makes it tighter. Hope this helps.

Try checking the heat shields on the exhaust pipe up front under the car. Alot of times they will loosen up, and begin to rattle. You can ither remove them, or try to clamp them.
Hope this helps!