Entropy loss

I am saying that a modern CPU (as opposed to an ancient Z80) already has a lot of complex unsynchronized clocks and delays within itself and within the memory controller. Just running a specific, carefully chosen set of instructions (that touch memory and perform arithmetic operations) is guaranteed to take slightly unpredictable time due to "beats" between these independent clocks, even if single threaded, even with interrupts disabled, and even if there is absolutely nothing else running.

So there is, in fact, an unintentionally-created hardware random number generator in every system which does not derive all clocks from a single crystal. There is software ("haveged") that repeatedly benchmarks this kind of instruction sequence using either a high-resolution timer or a cycle counter, and claims to extract entropy from that. Even without RDRAND, and it also works on many non-x86 CPUs. E.g., on mips 24kc in my TP-Link Archer C7, it gets around 200 kilobytes of random data per second. There is also a cryptoapi-based implementation of the same idea in the kernel, and a modern version of "rngd" also has this code. But there are also claims that the output is actually not random (despite passing statistical tests for randomness) and should not be trusted.