Investing.com - In a Wednesday note, Goldman Sachs (NYSE:GS) warned of the end of the current Bull Run, after years of stretched valuations.Christian Mueller-Glissmann, an analyst at Goldman Sachs, said "We are nearing the longest bull market for balanced equity/bond portfolios in over a century, boosted by a ‘Goldilocks’ backdrop of strong growth without inflation."Mueller-Glissmann added that the average valuation is expensive across all assets, and that strong valuations should become a ""speed limit for returns"".However, a Reuters survey of 13 fund managers, conducted in November, showed equity allocations in a model global portfolio rose to 57.2 percent from 56.7 percent – indicating that professional money managers are still piling up cash in the stock market.The current S&P 500 PE ratio is over 25, miles higher than the historical mean of 15.6 and median of 14.5 – suggesting stocks are indeed in rare territory as far as valuation goes. With 2017 coming to a close with a possible Fed rate hike in December, investors will be closely following how the beginning of 2018 unfolds for stocks and all financial assets.