Booming intermittent engine noise followed by flashing glow plug light

[yellowstep]

Hi all, long time lurker during my five-year ownership of the car and new member.

I've a 2008 Golf MK5 GT TDI 170, DSG, with 250k on the clock. Service items done by the book despite age/mileage.

It gradually loses coolant and smells of coolant/fumes at standstill - my guess is this a failed EGR cooler. There is also a hissing noise at idle. Very little visible steam from the exhaust, coolant sometimes a little black-tinged.

Aside from this, in the past 2k or so miles, my major concern:

- Intermittent, incredibly loud booming/droning noise coming from the engine bay
- Roughly every 150 or so miles and lasts five or so minutes
- Happens only on-throttle and in gear during these spells - off-throttle, it isn't there
- During these spells if I accelerate hard, over 3k revs I hear a scraping hollow metallic-type noise
- Will sometimes throw the flashing glow plug light and engine fault workshop
- Will sometimes go one worse if you continue to boot it and enter limp mode (I believe this is where the overboost code mentioned below was stored)
- When it does throw the light, the noise stops immediately
- Fuel consumption drops 10mpg average (is this noise tied to regens?)
- Feels down on power during these episodes
- Noisy spell remains if engine turned off mid-episode, left and turned on again
- Not as audible outside the car

NB

- Flywheel chatters a bit at idle but DSG box, changes and general behavior is and always has been fine
- Only experienced a DPF light once - and it was during this past 2k miles. Cleared on a motorway run and not seen since. I do a lot of long-distance driving
- Codes: one for overboost, cleared and has yet to return
- Engine light on for a dud Glow Plug
- Car idles nicely - not noisy or lumpy
- No issues starting, even with a Glow Plug out

I've scoured all over the place and found one thread on another forum that mentioned some of these symptoms but no real resolution, nor from the specialist!

Could it be the fault of a bad EGR cooler?

Help would be much appreciated! I don't want to give up on 250k miles

[VAG-Abound]

It may sound crazy but I tracked down what was causing noise inour engine by lashing some USB games console microphones in the engine bay. Long leads back into cabin to record on laptop. Stereo gives a remarkanle sense of being in the engine!