Is the plug firm or loose in the wall, like does it meet resistance as the prongs push the outlets contacts apart and slide through them or does it just slip in only to fall out if you sneeze too hard?
If its a good firm contact, then yeah Id 99.9999% go with it was just the input caps charging (and standby circuitry coming online). ie, nothing to worry about, normal stuff every piece of electronics, especially with large input caps, does when being connected to mains supply. It just so happened to be maybe that day had low enough humidity that you just happened to hear some of the static sizzle or some piezoelectric hum from the caps suddenly sucking juice (which they do at ridiculously high current just over a very short period of time). However if there is a switch on the PSU do try to remember to have it in the off position before connecting it to the mains supply. Its not necessarily dangerous or harmful per se, just a good SoP habit to integrate into your workflow.
If the plug feels almost no resistance and like it is barely holding on to the outlet, or can fall out under its own weight or with the slightest tug, especially if the sound is constant or intermittent but effectively continuous, then the sound could be contact arcing inside of the outlet housing due to the prongs of the plug and the contacts inside the outlet not being in firm contact with each other. !!!THIS IS DANGEROUS AND A FIRE HAZARD AS ARCING CAUSES HEAT BUILD UP WHICH CAN CAUSE PLASTIC TO MELT AND/OR OUTGAS WHICH CAN THEN BE IGNITED BY THE ARC!!! Or simply melt the insulation enough for current to flow between the hot wire and neutral low enough that the breaker doesn't pop but still plenty hot enough to light things around it on fire either from arcing or just from the heat itself. Not to mention if your hot and your neutral can get together where they're not supposed to chances are there's an electrocution hazard there too. Breakers (should) break instantly in a dead short scenario, but a 15A can happily pass 16A of current for hours if the current raises slowly enough. If the breaker is in an exposed panel where air can circulate around it thus providing some cooling to the breaker, it might never pop only passing one amp over its rating. Breakers dont operate how people think they do in non-dead short fault scenarios. All and all, no bueno.
The 'dad fix' is to just squeeze the prongs inward a little to grip the inner contacts a little better and 'hug' the center bit of the outlet preventing the plug from falling out. HOWEVER, be careful as you do not want to deform the outlet contacts in such a way that they dead short or arc internally. There is electrical code that dictates how close the bus bars that make up the contacts inside of an outlet are allowed to be to each other, not that you really have to worry about it beyond a potential fire hazard but the point is it's technically possible so there are electrical appliance standards that dictate bus bar spacing. This is a small trick up sleeve temp fix, only use a tiny bend, don't go crazy angling the prongs inward to revive an old tired outlet.
If you do have an old tired outlet, you have two options: either just order a new outlet and replace the old one (but remember unless you have an earth ground to connect the new outlet to, replacing two prong plug with a three-prong plug WILL NOT give you the safety of that ground lug and is considered illegal according to safety and building codes in a number of places), or if you like to tinker you could try your hand at disassembling the old socket (AFTER SHUTTING OFF POWER TO THAT OUTLET AND REMOVING IT FROM THE SUPPLY WIRING, OF COURSE!!! Use wire nuts to temporarily protect the bare mains wires if you need to have that circuit on, but have the breaker off when you're messing around with the wires and the wall box. It's quick, it's easy, replacing an old tired plug is not worth electrocuting yourself over, just turn the damn breaker off...) and bending the bus bar contacts back into a like new factory position. You can also take some time to clean the contact faces if they're dirty. Personally I would vote for just go get a new plug to replace an old tired one, but then take the old tired one apart to see how they're assembled and if you can clean it and bend the prongs back into a good position. Supposing the casing doesn't fall apart as you get it open, as some production processes will cause and in which case just throw it away when you're done looking at it, they're pretty foolproof in how they go together, a machine literally just drops the parts into the plastic casings and then pops a cover on. There's no need to force it if you can't just drop the bus bars back in and pop the cover on something's not in the right place. And do make sure to test it with a multimeters continuity tester to ensure there is no current path where there shouldn't be (hot to neutral hot to ground neutral to ground) if you're new to this kind of tinkering. I'm not new to this kind of tinkering and I still do it just in case. When wiring up the new plug, the smaller blade is the hot (black for US wiring), the wide blade is the neutral (white, US), and green or bare is the ground lug. YES, FOR ELECTRICAL SAFETY POLARIZATION MATTERS!!! DO NOT MIX UP THE HOT AND NEUTRAL ON POLARIZED PLUGS! Polarized plugs are those plugs where one blade is wider than the other, for non-polarized it doesn't matter as typically the first component in the circuit is a FULL BRIDGE RECTIFIER that converts AC to DC effectively eliminating the reasons for polarization concerns (FUN FACT: ~120Vac rectifies to ~170Vdc).
Now, if you're able to fix the old tired outlet you can just throw it in the utility drawer for the next time one of the outlets in the house needs to be replaced, as a temporary fix. Just don't push your luck outlets are cheap if it's not near factory perfect after you're done tinkering with it unless you are already familiar with mains power safety it's best not to trust it. Just get a new one. House fires suck.
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