Hot rubber smell with very heavy engine loads

I've noticed a mild smell of hot rubber, confirmed by a passenger, in my 04 MT hatchback (has ~125k miles) when driving up very steep, long hills at 45-60 mph, 3-4k rpm, and high (10k ft) elevation. The smell only comes when under these extreme loads. I recently replaced the timing belt and water pump, don't recal noticing the smell before the job but also only went skiing once before doing it and may have just passed a smell off at the time as coming from a semi. Any ideas or similar experiences? Thanks!