Here are my $.02 cents when it comes to M62 engines. In all of my M62 equipped BMWs when even ever I got to the point of resealing the timing covers I have always gone in and replaced the guides and chain for future piece of mind. My X5 started to exhibit a whirring sound at 99K miles and I thought it was one of the tensioners or bearings on the alternator, ac compressor and come to find out it was the beginning of guide failure. I like to have piece of mind that where ever I drive my BMWs they are mechanically sound. I could have gone a few more thousands of miles before changing them but I was already half way in there. The guides are gonna fail and I much rather be proactive then reactive to the situation when it occurs. This is a known concern with the M62 engines in the BMW X5.