I'm reading these CPUs are not hard to cool. Even so, I imagine that on a hot day that stock cooler will eventually draw attention to itself. I was thinking of a Noctua NH-U9S but would that just be wasting money?

Install the stock cooler. If you can hear it, then get an aftermarket one.

That would be the practical thing to do of course.

On a somewhat related note, interesting info here which suggest that, with an i7-8700, an aftermarket cooler has a marked positive effect on the all-core turbo frequency. Not so on the 8400, but interesting non the less.

I would assume Intel stock cooler may be quiet (at low-ish loads) but not at higher loads and rarely silent.

Any recommendations for a good value Heatsink/Fan that can cool an i5-8400 65W silently?

Cooler Master Hyper 212 EVO!

The fan on the evo is not that great, the cooler is alright nothing to brag about, but i would invest $6 more and get the Scythe Kotetsu, better fan, better mounting, more robust cooler and reviewed and recommended by SPCR Scythe Kotetsu CPU Cooler: A Compact King

Thanks. The review makes it sound very good indeed.
Not many stockists in the UK but I can get it for £44 - £47 which I'm happy to pay for if required.
I started a thread on coolers today which is why I found this one and others have recommended the Thermalright Macho Rev. B, Arctic Freezer i32 and Arctic Freezer 33 Plus.
I'll do a bit more research as I do have a nagging doubt about the Hyper 212 EVO just in case the fan noise disappoints.

Bit of a late follow up...

I tried the Intel stock cooler. It is the worst Intel stock cooler I have ever encountered, all the way back to the very first LGA775. Horrible, acute growling sound starting somewhere under 1000rpm. Gaming workload drives it to 1300-1600 where it is easily heard over ambient game audio (never mind rendering workload). I thought I got a lemon so I grabbed another from work: exactly the same! One interesting observation is that when held in the hand the growl is absent, but as soon as the assembly is even just placed on the table, it is manifest (I assume it has a real sloppy bearing who's rattle is damped when hand held, but amplified when set on any rigid surface).

Really disappointing, even for a bundled cooler.

Just trying to decide now between the NH-U9S and NH-U12S. The 9 is already pushing my spending comfort level. If I er on the high side and guess that it might have to turn 1300rpm or so under gaming load, looking at Mike's review from way back I'm thinking I could expect 18ish dB @ that speed. Sound reasonable (no pun intended)?

From my usual supplier the U9S is relatively expensive at around £50; the U12S is only £5 more. On this basis I would strongly recommend you go for the U12S. If you want to spend less money then from the same source is the Thermalright Macho Rev. B at £40.

For a non-overclocked CPU like the i5-8400, there will be no difference in performance between the stock cooler and an aftermarket cooler.

However, noise may be a concern regardless.

A budget aftermarket cooler with a quiet fan (i.e. Be Quiet Pure Rock Slim) is a good option.

If your budget is tight, look for the Scythe Kotetsu $35, nothing better around that pricepoint, if you have some time read SPCR Scythe Kotetsu CPU Cooler: A Compact King.

Now if you are on the UK, then go with Lodestar advice, pricing on scythe is not as cheap, Thermalright Macho Rev.B £39.98

PS. Really want to try out the NH-U12v2 with the twin NF-A12x25, it seems it performs as good as the NH-D15 on a smaller factor.

I re-read the SPCR reviews for two Noctuas. Although it was a long time ago, seems Lawrence wasn't overjoyed with the sound of the NF-F12 fan whereas Mike sounded pretty impressed with the NF-A9.

Looking at these results, it seems the 12cm has only a ~2degree advantage at 17dBA (though that advantage does get better by the time we get to 13-14dB). Do I read that right?

At 12dB there is an 8C difference which is significant. The U9S is better if you add a second fan but even then it is still 6C behind at 13-14dB. And of course buying a second fan would put the overall cost well past that of the U12S.

In my experience the speed range of the NF-F12 in practice can be around 300 at idle to about 700 rpm under load conditions. Higher ambient temperatures could make a difference, so would overclocking. I don't think you can deal with the CPU cooler in isolation you do need to take account of what support it is getting from the case fans. To keep CPU temperatures and fan speeds in check I recommend that the exhaust fan is replaced by a PWM fan linked to the CPU fan speed either through a PWM splitter cable, PWM Y cable or for some Asus motherboards the CPU_OPT PWM fan header. If there is a second exhaust position, such as top rear I would also advise that that be filled with another PWM fan linked in a similar way.

I decided to try the U9S, as much for its compactness as anything else. I've only farted around with it for a few hours but so far its seems a stellar upgrade to the Intel stock.

At idle its holding the CPU 15-20C over ambient @ <300rpm. Gaming load the fan is ~400rpm with the CPU ~35C over ambient. Blender rendering still just 600rpm, 40ish over ambient. Only on the small FFT P95 does it hit an audible 1300.

This is in a Define C with the 2 stock Fractal fans at the front, a Redux S12 at the back, all running at or slightly above their respective min rpm.

I used some old (very old) Arctic Silver 3 I had stored, just because I was curious to see how it would work. I'll probably redo with AS5 at some point and compare.