Braking is harder/softer in line with the whump sound.
TRP calliper brakes on aluminium rims.
Ideas?
have you checked for true and consistent rim width on a truing stand ? would do with tire on and off could be a soft spot
Ive got a noise a bit like that and the rim has a slight bulge in it.
time to get that front wheel trued up.
I’m sure you have already inspected the rim but do give it a thorough inspection if you haven’t already. Once on a 600km ride in the dead of night I hit a pothole and started to experience what you described. When the morning came I discovered my rim was cracked & it was a chilling moment to think about all the descending done on a dodgy cracked rim.
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Yup, rim issue seems most likely. Inspect the joint (usually on the opposite side to the stem hole). Many of these joints are pinned but not welded. These joints are often smooth and hard to feel, but they can lead to the sound and pulsing you mention if the transition is no longer smooth.
That or any other dent or major localized deviation are reason the braking will vary along the circumference.
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Sounds good, I’ll swap out the front wheel see if it goes away. And then check the rim 
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Yep.
Swapped over wheel, no whumping 
Don’t have a truing station though and eyeballing it I can’t see any obvious imperfections. Any other non-paid for steps I can take?
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Put it back in your frame and tighten your brake pads it’ll make it easier to spot if its a little out of true (rubbing the brakes at a point)…
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Zip tie around the brake lever to get the pads as close as you choose and hold them there without having to actually adjust them.
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Is there any other possible cause?
I mean, because if it is out of true then am I realistically going to retrue it myself? Ie it needs to go to the shop anyway?
It’s a Campag Bullet 80 if that makes a difference to whether we are talking spoke tuning or something else.
Only ridden outside a couple of times a year since 2017.
A bulge in the rim perhaps or some other sort of rim damage; sometimes you can take an adjustable spanner to sort it if they are alloy rims. Trueing is not desperately hard if its minor but spoke tension would worry me if it needs a major true.
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Contamination on part but not all of the braking surface? Either something sticky or slippery that makes part of the rim circumfrence grab more or less than the rest?
The likelihood of that happening in a way that the substance doesn’t end up getting dragged onto the rest of the rim relatively quickly is pretty slim though. Guessing you’d probably remember encountering something unique enough to do that.
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Great thought on the possible surface contamination. That is worth good clean. I use simple window cleaner and a paper towel or kitchen sponge to wipe the entire braking surface on both sides.
That is also great time to inspect and may show (or let you feel) for any bad or bent surfaces on the rim too (including the rim joint).
And as mentioned, a quick “true” inspection can be done in the frame with the brakes as a decent guide.
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I shall commence scrubbing.
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Try it the other way round - sometimes the join is slightly out of line and can catch one way more than the other. I’ve got it on my commute bike front wheel…which is a dynamo so can’t be swapped around 
In that case the pads will eventually wear the edge off…
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