Unfortunate mistake but I guess it’s a learning experience
My claim to fame is putting oil in the engine having forgotten to put the sump plug in
Anyway the noise could be from any part of the engine and depends on what you’ve actually done. I’d guess, if you took the heads off and cleaned the parts, you’d have used assembly lube before putting them back together, so they should last better than if you just squirted a little oil on them during assembly.
But the actual noise could be coming from the bits you’ve cleaned, like the cam followers, pushrods, rockers or valve stem tops, timing chain idlers, etc. They could also be coming from any other part of the engine that you haven’t disturbed but requires oil to stop any metal on metal contact.
I’d suggest whatever damage is done is done. What you do now is limited unless you want to strip down the engine and check and measure all running surfaces.
So right now I’d look at some oil additive like Liqui Moly MOS2 anti friction treatment and some new high quality oil and filter. If you’re lucky the extra friction reducing compounds in the additive will make the squeak go away or become less noticeable.
Note that I’ve never used such additives myself as I do more frequent oil changes than specified in the service schedule. As such I can’t say how well such an additive would work. But what have you got to loose now?
Also, don’t discount some air leak from a dislodged hose caused when lifting the engine, sometimes squeak and hisses can sound similar. The sound could also be coming from front engine drive belt tensioner or the idler pulleys. Best to use a mechanics stethoscope to better localise the squeak (but try the additive first so you don’t do more mechanical damage if it is indeed internal).
As a parting shot, I’ve seen videos of people running engines after draining all oil from them, and surprisingly, they can idle for hours and hours. Guess that’s why I’m sceptical of some anti wear additive promos that do similar as a way of showing their worth. So if you used assembly lube when putting things back together and only idled from 5 minutes, the probability is that the noise may indeed be something external (if you’re lucky). Still, an additive is worth a try before further fault finding (since you don’t want to make any damage worse than it is).
Good luck.
