I currently have two Noiseblocker M12-S2 fans for air intake and noticed that they are louder for intake than exhaust. Are there any alternatives that are quieter for intake than the Noiseblocker fans @600 rpm?

You can try to cut the grills

And if that doesn't work try two undervolted nexus fans.

I doubt Nexus will be more silent than M12-S2 at similar speed, those two fans are damn good. I suspect a fan with lower static pressure may help, perhaps a Scythe Slipstream ?

I agree. But if cutting the grill doesn't work you need to do something.

Most likely the Noisblockers sustain higher static pressure than the Nexus. Actually you would expect higher static pressure fans to generate less noise, not more... presuming that the fans are facing backpressure.

Maybe the next step is actually to reduce the speed of the noise blockers.

Well, I'd rather not cut the grills

I tried reducing the speed of the M12-S2 further, but couldn't get them reliably lower than about 500rpm to 550rpm. Would a fan like Scythe Gentle Typhoon @500 be better?

What is static pressure? What does it mean for the fan, and why is higher pressure more quiet?

When fans face back pressure (static pressure), different fans respond differently.

Some fans that "have high static pressure" handle the back pressure easily and continue to move CFMs of air.

Other fans handle the back pressure poorly. Instead of moving air the flan blades luff and stall. Generally this increases the noise of the fan. Such fans are said to "have low static pressure".

All things equal (which they never are) you would expect a "high static pressure" fan to operate more quietly against high back pressure (and also deliver more CFM while doing so)... and you would expect a "low static pressure" fan to be able to deliver more CFM under low back pressure... and do to so more quietly than a "high static pressure" fan.

Noiseblockers have a reputation for being quiet at all pressures and for handling high back pressure just fine.

If the fans are making noise, you need to experiment with different fans. Nexus is a good choice. So are Noctua fans if you operate them at low voltage. It may be you just got a bad sample of noiseblockers.

All you can do is experiment.

You can always try an M12 S1 which is a 750rpm (in my experience is more like 850rpm), might go lower than S2 undervolting it (personally i havent tried though).

Another fan that can Ttry is Scythe Slipstream PWM (this is if you have a mobo that can control PWM fans), in my case i was able to lower it to 250rpm where it barely pushes any air but it can be ramp up depending on the conditions.

If all fails, get BeQuiet!'s (no matter which way you look at it, they're the best, sorry). It's the only fan that was bearable for me as a side or bottom intake. Also take not that (side) intake fan is the most difficult position to get a quiet solution for.