Something to consider if you have access to one. Do you know anyone who has a car lift that could help you for half an hour or so. If so, raise it enough to get the wheels off the ground, have someone 'drive' it at the 5-10mph speed you mention whilst you listen in and around the engine bay, as you are familiar with the sound.
Go through the various examples of when it's likely to happen, and see if you can pinpoint it any better. You should stand a better chance, as you would be eliminating wind and tyre noise from your surroundings. You also have the opportunity of being able to move around the car both left and right, and from above and below.
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs - Replaced alarm horn, Replaced Coolant Temp sensor, Updated RNS-D firmware, Installed Reversing Camera and Digital TV, Dashcam Installation, Retrofitted Rear Blinds, Auto-Dimming Rear View Mirror, Chrome Boot Struts
Sold!  2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs - Fan Jet Windscreen Washers, Retrofitted Electric Side Blinds
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